diff --git a/tsc/internal/checker/checker.go b/tsc/internal/checker/checker.go
index 143d1b4b5596b..8dd981cafba08 100644
--- a/tsc/internal/checker/checker.go
+++ b/tsc/internal/checker/checker.go
@@ -17806,6 +17806,12 @@ func (b *keyBuilder) spill() {
 	b.inlineLength = 0
 }
 
+func (b *keyBuilder) reserve(size int) {
+	if b.overflowBuffer != nil || b.inlineLength+size > len(b.inlineBuffer) {
+		b.overflowBuffer = slices.Grow(b.overflowBuffer, b.inlineLength+size)
+	}
+}
+
 func (b *keyBuilder) writeByte(c byte) {
 	if b.inlineLength == len(b.inlineBuffer) {
 		b.spill()
@@ -17854,6 +17860,7 @@ func (b *keyBuilder) writeType(t *Type) {
 }
 
 func (b *keyBuilder) writeTypes(types []*Type) {
+	b.reserve(8 + 4*len(types))
 	b.writeInt(len(types))
 	for _, t := range types {
 		b.writeType(t)
diff --git a/tsc/internal/checker/checker_test.go b/tsc/internal/checker/checker_test.go
index 0ca836c03b1ea..31cea19751525 100644
--- a/tsc/internal/checker/checker_test.go
+++ b/tsc/internal/checker/checker_test.go
@@ -157,6 +157,26 @@ func BenchmarkCheck(b *testing.B) {
 	}
 }
 
+func BenchmarkCheckFreshProgram(b *testing.B) {
+	fs := bundled.WrapFS(osvfs.FS())
+	rootPath := tspath.RootedDirectoryPathFromAbsolute(filepath.Join(repo.TestDataPath(), "fixtures/compiler"))
+	host := compiler.NewCompilerHost(fs, bundled.LibPath(), nil, nil, nil)
+	parsed, errors := tsoptions.GetParsedCommandLineOfConfigFile(rootPath.ResolveFile("tsconfig.json"), &core.CompilerOptions{}, nil, fs, nil)
+	assert.Equal(b, len(errors), 0, "Expected no errors in parsed command line")
+	ctx := b.Context()
+
+	b.ReportAllocs()
+	for b.Loop() {
+		program := compiler.NewProgram(compiler.ProgramOptions{Config: parsed, Host: host})
+		program.BindSourceFiles()
+		c, _ := checker.NewChecker(program, nil)
+		for _, file := range program.GetSourceFiles() {
+			assert.Equal(b, len(c.GetDiagnostics(ctx, file)), 0)
+		}
+		assert.Equal(b, len(c.GetGlobalDiagnostics()), 0)
+	}
+}
+
 func TestTypeAllocationAcrossCheckerGrowth(t *testing.T) {
 	t.Parallel()
 	var content strings.Builder
diff --git a/tsc/internal/checker/types_test.go b/tsc/internal/checker/types_test.go
index 8b21b07dc8160..a914dd424f8ea 100644
--- a/tsc/internal/checker/types_test.go
+++ b/tsc/internal/checker/types_test.go
@@ -1,10 +1,15 @@
 package checker
 
 import (
+	"encoding/binary"
 	"runtime"
+	"strconv"
+	"strings"
 	"testing"
 	"unsafe"
 
+	"github.com/microsoft/TypeScript/tsc/internal/testutil/race"
+	"github.com/zeebo/xxh3"
 	"gotest.tools/v3/assert"
 )
 
@@ -73,3 +78,65 @@ func TestTypeHeaderSize(t *testing.T) {
 	t.Parallel()
 	assert.Equal(t, unsafe.Sizeof(Type{}), 16+3*unsafe.Sizeof((*Checker)(nil)))
 }
+
+func TestKeyBuilder(t *testing.T) {
+	t.Parallel()
+	for _, count := range []int{0, 1, 46, 47, 256, 4096} {
+		t.Run(strconv.Itoa(count), func(t *testing.T) {
+			t.Parallel()
+			types := make([]*Type, count)
+			var expected []byte
+			prefix := strings.Repeat("x", count)
+			expected = append(expected, prefix...)
+			expected = append(expected, 17)
+			expected = binary.LittleEndian.AppendUint32(expected, 123)
+			expected = binary.LittleEndian.AppendUint64(expected, uint64(count))
+			for i := range types {
+				types[i] = &Type{id: TypeId(i + 1)}
+				expected = binary.LittleEndian.AppendUint32(expected, uint32(i+1))
+			}
+			expected = append(expected, "suffix"...)
+			var builder keyBuilder
+			builder.writeString(prefix)
+			builder.writeByte(17)
+			builder.writeUint32(123)
+			builder.writeTypes(types)
+			builder.writeString("suffix")
+			assert.Equal(t, builder.hash(), CacheHashKey(xxh3.Hash128(expected)))
+			assert.Equal(t, builder.hash(), CacheHashKey(xxh3.Hash128(expected)))
+		})
+	}
+}
+
+func TestTypeListKeyAllocations(t *testing.T) { //nolint:paralleltest
+	if race.Enabled {
+		t.Skip("race instrumentation changes hashing allocations")
+	}
+	for _, count := range []int{8, 46, 47, 256, 4096} {
+		types := make([]*Type, count)
+		for i := range types {
+			types[i] = &Type{id: TypeId(i + 1)}
+		}
+		allocations := testing.AllocsPerRun(10, func() { getTypeListKey(types) })
+		expected := float64(0)
+		if count > 46 {
+			expected = 1
+		}
+		assert.Equal(t, allocations, expected, "type list length %d", count)
+	}
+}
+
+func BenchmarkTypeListKey(b *testing.B) {
+	for _, count := range []int{8, 46, 47, 256, 4096} {
+		b.Run(strconv.Itoa(count), func(b *testing.B) {
+			types := make([]*Type, count)
+			for i := range types {
+				types[i] = &Type{id: TypeId(i + 1)}
+			}
+			b.ReportAllocs()
+			for b.Loop() {
+				getTypeListKey(types)
+			}
+		})
+	}
+}