diff --git a/pkg/chipingress/batch/client.go b/pkg/chipingress/batch/client.go
index 93c9efd049..0ec9902611 100644
--- a/pkg/chipingress/batch/client.go
+++ b/pkg/chipingress/batch/client.go
@@ -14,6 +14,7 @@ import (
 	"go.opentelemetry.io/otel/attribute"
 	otelmetric "go.opentelemetry.io/otel/metric"
 	"go.uber.org/zap"
+	"google.golang.org/protobuf/encoding/protowire"
 	"google.golang.org/protobuf/proto"
 
 	"github.com/smartcontractkit/chainlink-common/pkg/chipingress"
@@ -36,18 +37,18 @@ type Client struct {
 	maxGRPCRequestSize      int // configured max, used for metrics/error reporting
 	effectiveMaxRequestSize int // maxGRPCRequestSize minus grpcFramingOverhead, used for splitting
 	cloneEvent              bool
-	maxConcurrentSends chan struct{}
-	batchInterval      time.Duration
-	maxPublishTimeout  time.Duration
-	messageBuffer      chan *messageWithCallback
-	stopCh             stopCh
-	log                *zap.SugaredLogger
-	callbackWg         sync.WaitGroup
-	shutdownTimeout    time.Duration
-	shutdownOnce       sync.Once
-	batcherDone        chan struct{}
-	started            bool
-	counters           sync.Map // map[seqnumKey]*atomic.Uint64 for per-(source,type) seqnum, cleared on Stop()
+	maxConcurrentSends      chan struct{}
+	batchInterval           time.Duration
+	maxPublishTimeout       time.Duration
+	messageBuffer           chan *messageWithCallback
+	stopCh                  stopCh
+	log                     *zap.SugaredLogger
+	callbackWg              sync.WaitGroup
+	shutdownTimeout         time.Duration
+	shutdownOnce            sync.Once
+	batcherDone             chan struct{}
+	started                 bool
+	counters                sync.Map // map[seqnumKey]*atomic.Uint64 for per-(source,type) seqnum, cleared on Stop()
 
 	metrics batchClientMetrics
 }
@@ -73,18 +74,18 @@ func NewBatchClient(client chipingress.Client, opts ...Opt) (*Client, error) {
 	c := &Client{
 		client:                  client,
 		log:                     zap.NewNop().Sugar(),
-		batchSize:               10,
+		batchSize:               100,
 		maxGRPCRequestSize:      10 * 1024 * 1024,
 		effectiveMaxRequestSize: 10*1024*1024 - grpcFramingOverhead,
 		cloneEvent:              true,
-		maxConcurrentSends: make(chan struct{}, 1),
-		messageBuffer:      make(chan *messageWithCallback, 200),
-		batchInterval:      100 * time.Millisecond,
-		maxPublishTimeout:  5 * time.Second,
-		stopCh:             make(chan struct{}),
-		callbackWg:         sync.WaitGroup{},
-		shutdownTimeout:    5 * time.Second,
-		batcherDone:        make(chan struct{}),
+		maxConcurrentSends:      make(chan struct{}, 1),
+		messageBuffer:           make(chan *messageWithCallback, 200),
+		batchInterval:           100 * time.Millisecond,
+		maxPublishTimeout:       5 * time.Second,
+		stopCh:                  make(chan struct{}),
+		callbackWg:              sync.WaitGroup{},
+		shutdownTimeout:         5 * time.Second,
+		batcherDone:             make(chan struct{}),
 	}
 
 	for _, opt := range opts {
@@ -322,17 +323,31 @@ func splitMessagesByRequestSize(messages []*messageWithCallback, maxRequestSize
 		return [][]*messageWithCallback{messages}
 	}
 
+	// A CloudEventBatch encodes its events as a repeated field, so the serialized
+	// request size is a fixed base (everything except the events) plus the
+	// independent contribution of each event. Accumulating per-event sizes keeps
+	// splitting O(n): previously each message re-serialized the whole growing
+	// batch via newBatchRequest -> proto.Size, costing O(n^2) bytes walked.
+	_, baseBytes := newBatchRequest(nil)
+
 	var batches [][]*messageWithCallback
+	// Size the first batch for the common case where everything fits in one
+	// request. Subsequent batches (only created when splitting) start small and
+	// grow, so we don't allocate a full-length backing array per split.
 	current := make([]*messageWithCallback, 0, len(messages))
+	currentBytes := baseBytes
 	for _, msg := range messages {
-		candidate := append(current, msg)
-		_, candidateBytes := newBatchRequest(candidate)
-		if len(current) > 0 && candidateBytes > maxRequestSize {
+		eventBytes := eventFieldSize(msg.event)
+		// Start a new batch once adding this event would exceed the limit, but
+		// always keep at least one event per batch (an oversized single event is
+		// rejected later in sendBatch).
+		if len(current) > 0 && currentBytes+eventBytes > maxRequestSize {
 			batches = append(batches, current)
-			current = []*messageWithCallback{msg}
-			continue
+			current = nil
+			currentBytes = baseBytes
 		}
-		current = candidate
+		current = append(current, msg)
+		currentBytes += eventBytes
 	}
 	if len(current) > 0 {
 		batches = append(batches, current)
@@ -340,6 +355,15 @@ func splitMessagesByRequestSize(messages []*messageWithCallback, maxRequestSize
 	return batches
 }
 
+// eventFieldSize returns the number of bytes a single event contributes to a
+// CloudEventBatch: the repeated events field tag plus the length-delimited
+// message. This matches proto.Size's accounting for that field exactly, so the
+// running total stays identical to proto.Size(batch).
+func eventFieldSize(event *chipingress.CloudEventPb) int {
+	const eventsFieldNumber = 1 // CloudEventBatch.events
+	return protowire.SizeTag(eventsFieldNumber) + protowire.SizeBytes(proto.Size(event))
+}
+
 func newBatchRequest(messages []*messageWithCallback) (*chipingress.CloudEventBatch, int) {
 	events := make([]*chipingress.CloudEventPb, len(messages))
 	for i, msg := range messages {
diff --git a/pkg/chipingress/batch/client_test.go b/pkg/chipingress/batch/client_test.go
index e698e75c61..0792a3f747 100644
--- a/pkg/chipingress/batch/client_test.go
+++ b/pkg/chipingress/batch/client_test.go
@@ -28,6 +28,10 @@ func TestNewBatchClient(t *testing.T) {
 		client, err := NewBatchClient(mocks.NewClient(t))
 		require.NoError(t, err)
 		assert.NotNil(t, client)
+		// Default batchSize is 100. Guard the default explicitly: it changes the
+		// gRPC payload size and events-per-callback for every caller that doesn't
+		// pass WithBatchSize, so an accidental revert should fail loudly.
+		assert.Equal(t, 100, client.batchSize)
 	})
 
 	t.Run("WithBatchSize", func(t *testing.T) {
@@ -1599,6 +1603,67 @@ func TestSplitMessagesByRequestSize(t *testing.T) {
 			assert.Len(t, batch, 1)
 		}
 	})
+
+	t.Run("eventFieldSize accumulation matches proto.Size for the whole batch", func(t *testing.T) {
+		msgs := []*messageWithCallback{
+			{event: largeTestEvent("a")},
+			{event: largeTestEvent("bb")},
+			{event: largeTestEvent("ccc")},
+		}
+		events := make([]*chipingress.CloudEventPb, len(msgs))
+		_, summed := newBatchRequest(nil)
+		for i, m := range msgs {
+			events[i] = m.event
+			summed += eventFieldSize(m.event)
+		}
+		// The incremental total used for splitting must equal the real
+		// serialized size; otherwise splits could under/over-count.
+		assert.Equal(t, proto.Size(&chipingress.CloudEventBatch{Events: events}), summed)
+	})
+
+	t.Run("every produced batch fits within the limit by proto.Size", func(t *testing.T) {
+		msgs := make([]*messageWithCallback, 0, 25)
+		for i := 0; i < 25; i++ {
+			msgs = append(msgs, &messageWithCallback{event: largeTestEvent(strconv.Itoa(i))})
+		}
+		// A limit that holds two events but forces several splits.
+		twoEvents := &chipingress.CloudEventBatch{Events: []*chipingress.CloudEventPb{msgs[0].event, msgs[1].event}}
+		maxRequestSize := proto.Size(twoEvents)
+
+		result := splitMessagesByRequestSize(msgs, maxRequestSize)
+		require.Greater(t, len(result), 1, "expected the batch to be split")
+
+		var total int
+		for _, batch := range result {
+			require.NotEmpty(t, batch)
+			events := make([]*chipingress.CloudEventPb, len(batch))
+			for i, m := range batch {
+				events[i] = m.event
+			}
+			assert.LessOrEqual(t, proto.Size(&chipingress.CloudEventBatch{Events: events}), maxRequestSize)
+			total += len(batch)
+		}
+		assert.Equal(t, len(msgs), total, "no events dropped across splits")
+	})
+
+	t.Run("event larger than the limit gets its own batch and is not dropped", func(t *testing.T) {
+		big := largeTestEvent("big")
+		// A limit below a single event's contribution forces every event into its
+		// own batch. Splitting keeps the event (sendBatch is responsible for
+		// rejecting it against maxGRPCRequestSize), never drops it.
+		maxRequestSize := eventFieldSize(big) - 1
+		msgs := []*messageWithCallback{
+			{event: big},
+			{event: largeTestEvent("small")},
+		}
+
+		result := splitMessagesByRequestSize(msgs, maxRequestSize)
+		require.Len(t, result, 2, "each oversized event must land in its own batch")
+		for _, batch := range result {
+			require.Len(t, batch, 1)
+		}
+		assert.Same(t, big, result[0][0].event, "oversized event preserved, not dropped")
+	})
 }
 
 func BenchmarkSendBatch(b *testing.B) {
diff --git a/pkg/chipingress/batch/split_bench_test.go b/pkg/chipingress/batch/split_bench_test.go
new file mode 100644
index 0000000000..b5c3a467cf
--- /dev/null
+++ b/pkg/chipingress/batch/split_bench_test.go
@@ -0,0 +1,85 @@
+package batch
+
+import (
+	"strconv"
+	"testing"
+
+	"github.com/smartcontractkit/chainlink-common/pkg/chipingress"
+	"google.golang.org/protobuf/proto"
+)
+
+// splitMessagesByRequestSizeQuadratic is the previous implementation, kept here
+// only as a benchmark baseline. It re-serializes the whole growing batch on
+// every message (O(n^2) bytes walked), whereas splitMessagesByRequestSize
+// accumulates per-event sizes (O(n)).
+func splitMessagesByRequestSizeQuadratic(messages []*messageWithCallback, maxRequestSize int) [][]*messageWithCallback {
+	if len(messages) == 0 {
+		return nil
+	}
+	if maxRequestSize <= 0 {
+		return [][]*messageWithCallback{messages}
+	}
+
+	var batches [][]*messageWithCallback
+	current := make([]*messageWithCallback, 0, len(messages))
+	for _, msg := range messages {
+		candidate := append(current, msg)
+		_, candidateBytes := newBatchRequest(candidate)
+		if len(current) > 0 && candidateBytes > maxRequestSize {
+			batches = append(batches, current)
+			current = []*messageWithCallback{msg}
+			continue
+		}
+		current = candidate
+	}
+	if len(current) > 0 {
+		batches = append(batches, current)
+	}
+	return batches
+}
+
+func benchSplitMessages(n int) []*messageWithCallback {
+	msgs := make([]*messageWithCallback, n)
+	for i := range msgs {
+		msgs[i] = &messageWithCallback{event: largeTestEvent(strconv.Itoa(i))}
+	}
+	return msgs
+}
+
+// benchSplitMaxRequestSize returns a limit that holds ~perBatch events, so the
+// input is split into many sub-batches (the case where the quadratic cost bites).
+func benchSplitMaxRequestSize(msgs []*messageWithCallback, perBatch int) int {
+	if perBatch > len(msgs) {
+		perBatch = len(msgs)
+	}
+	events := make([]*chipingress.CloudEventPb, perBatch)
+	for i := 0; i < perBatch; i++ {
+		events[i] = msgs[i].event
+	}
+	return proto.Size(&chipingress.CloudEventBatch{Events: events})
+}
+
+// BenchmarkSplitMessagesByRequestSize compares the linear (current) and
+// quadratic (old) splitters across batch sizes.
+//
+//	go test ./pkg/chipingress/batch -bench BenchmarkSplitMessagesByRequestSize -benchmem -run '^$'
+func BenchmarkSplitMessagesByRequestSize(b *testing.B) {
+	for _, n := range []int{10, 100, 1000} {
+		msgs := benchSplitMessages(n)
+		maxRequestSize := benchSplitMaxRequestSize(msgs, 10)
+
+		b.Run("linear/n="+strconv.Itoa(n), func(b *testing.B) {
+			b.ReportAllocs()
+			for i := 0; i < b.N; i++ {
+				_ = splitMessagesByRequestSize(msgs, maxRequestSize)
+			}
+		})
+
+		b.Run("quadratic/n="+strconv.Itoa(n), func(b *testing.B) {
+			b.ReportAllocs()
+			for i := 0; i < b.N; i++ {
+				_ = splitMessagesByRequestSizeQuadratic(msgs, maxRequestSize)
+			}
+		})
+	}
+}