diff --git a/tsc/internal/checker/checker.go b/tsc/internal/checker/checker.go
index 10ddb3c0c88b4..5f650b74fa71b 100644
--- a/tsc/internal/checker/checker.go
+++ b/tsc/internal/checker/checker.go
@@ -920,7 +920,7 @@ type Checker struct {
 	ctx                                         context.Context
 	packagesMap                                 map[string]bool
 	activeMappers                               []*TypeMapper
-	activeTypeMappersCaches                     []map[CacheHashKey]*Type
+	activeTypeMappersCaches                     []clearableMap[*Type]
 	ambientModulesOnce                          sync.Once
 	ambientModules                              []*ast.Symbol
 	withinUnreachableCode                       bool
@@ -22624,16 +22624,15 @@ func (c *Checker) instantiateTypeWithAlias(t *Type, m *TypeMapper, alias *TypeAl
 		return c.errorType
 	}
 	index := c.findActiveMapper(m)
+	var key CacheHashKey
 	if index == -1 {
+		// A newly pushed mapper has an empty cache, and the result is not cached below.
 		c.pushActiveMapper(m)
-	}
-	var b keyBuilder
-	b.writeType(t)
-	b.writeAlias(alias)
-	key := b.hash()
-	cache := c.activeTypeMappersCaches[core.IfElse(index != -1, index, len(c.activeTypeMappersCaches)-1)]
-	if cachedType, ok := cache[key]; ok {
-		return cachedType
+	} else {
+		key = getActiveMapperCacheKey(t, alias)
+		if cachedType, ok := c.activeTypeMappersCaches[index].get(key); ok {
+			return cachedType
+		}
 	}
 	c.TotalInstantiationCount++
 	c.instantiationCount++
@@ -22642,13 +22641,25 @@ func (c *Checker) instantiateTypeWithAlias(t *Type, m *TypeMapper, alias *TypeAl
 	if index == -1 {
 		c.popActiveMapper()
 	} else {
-		cache[key] = result
+		c.activeTypeMappersCaches[index].set(key, result)
 	}
 	c.instantiationStack[len(c.instantiationStack)-1] = nil
 	c.instantiationStack = c.instantiationStack[:len(c.instantiationStack)-1]
 	return result
 }
 
+// Without an alias the key is just the type ID, which avoids hashing. Hashed keys have
+// a nonzero high half with overwhelming probability, so the two kinds do not collide.
+func getActiveMapperCacheKey(t *Type, alias *TypeAlias) CacheHashKey {
+	if alias == nil {
+		return CacheHashKey{Lo: uint64(t.id)}
+	}
+	var b keyBuilder
+	b.writeType(t)
+	b.writeAlias(alias)
+	return b.hash()
+}
+
 func (c *Checker) getCircularTypeNames() []string {
 	typeCounts := make(map[*Type]int)
 	var circularTypeNames []string
@@ -22674,13 +22685,10 @@ func (c *Checker) pushActiveMapper(mapper *TypeMapper) {
 
 	lastIndex := len(c.activeTypeMappersCaches)
 	if cap(c.activeTypeMappersCaches) > lastIndex {
-		// The cap may contain an empty map from popActiveMapper; reuse it.
+		// The cap may contain an empty cache from popActiveMapper; reuse it.
 		c.activeTypeMappersCaches = c.activeTypeMappersCaches[:lastIndex+1]
-		if c.activeTypeMappersCaches[lastIndex] == nil {
-			c.activeTypeMappersCaches[lastIndex] = make(map[CacheHashKey]*Type, 1)
-		}
 	} else {
-		c.activeTypeMappersCaches = append(c.activeTypeMappersCaches, make(map[CacheHashKey]*Type, 1))
+		c.activeTypeMappersCaches = append(c.activeTypeMappersCaches, clearableMap[*Type]{})
 	}
 }
 
@@ -22688,9 +22696,9 @@ func (c *Checker) popActiveMapper() {
 	c.activeMappers[len(c.activeMappers)-1] = nil
 	c.activeMappers = c.activeMappers[:len(c.activeMappers)-1]
 
-	// Clear the map, but leave it in the list for later reuse.
+	// Clear the cache, but leave it in the list for later reuse.
 	lastIndex := len(c.activeTypeMappersCaches) - 1
-	clear(c.activeTypeMappersCaches[lastIndex])
+	c.activeTypeMappersCaches[lastIndex].clear()
 	c.activeTypeMappersCaches = c.activeTypeMappersCaches[:lastIndex]
 }
 
@@ -22699,8 +22707,8 @@ func (c *Checker) findActiveMapper(mapper *TypeMapper) int {
 }
 
 func (c *Checker) clearActiveMapperCaches() {
-	for _, cache := range c.activeTypeMappersCaches {
-		clear(cache)
+	for i := range c.activeTypeMappersCaches {
+		c.activeTypeMappersCaches[i].clear()
 	}
 }
 
diff --git a/tsc/internal/checker/clearablemap.go b/tsc/internal/checker/clearablemap.go
new file mode 100644
index 0000000000000..08564219e70b7
--- /dev/null
+++ b/tsc/internal/checker/clearablemap.go
@@ -0,0 +1,87 @@
+package checker
+
+import "math/bits"
+
+// clearableMap is a hash table keyed by 128-bit keys that can be cleared in constant time. It
+// is used for short-lived caches that are cleared and reused many times: a Go map takes time
+// proportional to its capacity to clear, which stays large once it has grown. Here each entry
+// records the generation it was written in, and clearing starts a new one.
+type clearableMap[V any] struct {
+	entries    []clearableMapEntry[V]
+	generation uint32
+	count      int
+	shift      uint8
+}
+
+type clearableMapEntry[V any] struct {
+	key        CacheHashKey
+	value      V
+	generation uint32
+}
+
+func (m *clearableMap[V]) slot(key CacheHashKey) int {
+	return int(((key.Lo ^ key.Hi) * 0x9e3779b97f4a7c15) >> m.shift)
+}
+
+func (m *clearableMap[V]) get(key CacheHashKey) (V, bool) {
+	var zero V
+	if m.count == 0 {
+		return zero, false
+	}
+	mask := len(m.entries) - 1
+	for i := m.slot(key); ; i = (i + 1) & mask {
+		e := &m.entries[i]
+		if e.generation != m.generation {
+			return zero, false
+		}
+		if e.key == key {
+			return e.value, true
+		}
+	}
+}
+
+func (m *clearableMap[V]) set(key CacheHashKey, value V) {
+	if (m.count+1)*4 > len(m.entries)*3 {
+		m.grow()
+	}
+	mask := len(m.entries) - 1
+	for i := m.slot(key); ; i = (i + 1) & mask {
+		e := &m.entries[i]
+		if e.generation != m.generation {
+			*e = clearableMapEntry[V]{key: key, value: value, generation: m.generation}
+			m.count++
+			return
+		}
+		if e.key == key {
+			e.value = value
+			return
+		}
+	}
+}
+
+func (m *clearableMap[V]) grow() {
+	old := m.entries
+	oldGeneration := m.generation
+	size := max(len(old)*2, 8)
+	m.entries = make([]clearableMapEntry[V], size)
+	m.shift = uint8(64 - bits.TrailingZeros(uint(size)))
+	m.generation = 1
+	m.count = 0
+	for i := range old {
+		if e := &old[i]; e.generation == oldGeneration {
+			m.set(e.key, e.value)
+		}
+	}
+}
+
+func (m *clearableMap[V]) clear() {
+	if m.count == 0 {
+		return
+	}
+	m.count = 0
+	m.generation++
+	if m.generation == 0 {
+		clear(m.entries)
+		m.generation = 1
+	}
+}
diff --git a/tsc/internal/checker/inference.go b/tsc/internal/checker/inference.go
index 02c7d009b9ec5..9fadf41334ef7 100644
--- a/tsc/internal/checker/inference.go
+++ b/tsc/internal/checker/inference.go
@@ -26,7 +26,7 @@ type InferenceState struct {
 	bivariant         bool
 	expandingFlags    ExpandingFlags
 	propagationType   *Type
-	visited           map[InferenceKey]InferencePriority
+	visited           clearableMap[InferencePriority]
 	sourceStack       []*Type
 	targetStack       []*Type
 	next              *InferenceState
@@ -42,7 +42,7 @@ func (c *Checker) getInferenceState() *InferenceState {
 }
 
 func (c *Checker) putInferenceState(n *InferenceState) {
-	clear(n.visited)
+	n.visited.clear()
 	*n = InferenceState{
 		inferences:  n.inferences[:0],
 		visited:     n.visited,
@@ -344,15 +344,15 @@ func (c *Checker) inferFromContravariantTypesIfStrictFunctionTypes(n *InferenceS
 // such that we would go on inferring forever, even though we would never infer
 // between the same pair of types.
 func (c *Checker) invokeOnce(n *InferenceState, source *Type, target *Type, action func(c *Checker, n *InferenceState, source *Type, target *Type)) {
-	key := InferenceKey{source: source.id, target: target.id, priority: n.priority, contravariant: n.contravariant, bivariant: n.bivariant}
-	if status, ok := n.visited[key]; ok {
+	key := CacheHashKey{
+		Lo: uint64(source.id)<<32 | uint64(target.id),
+		Hi: uint64(uint32(n.priority))<<2 | uint64(core.IfElse(n.contravariant, 2, 0)) | uint64(core.IfElse(n.bivariant, 1, 0)),
+	}
+	if status, ok := n.visited.get(key); ok {
 		n.inferencePriority = min(n.inferencePriority, status)
 		return
 	}
-	if n.visited == nil {
-		n.visited = make(map[InferenceKey]InferencePriority)
-	}
-	n.visited[key] = InferencePriorityCircularity
+	n.visited.set(key, InferencePriorityCircularity)
 	saveInferencePriority := n.inferencePriority
 	n.inferencePriority = InferencePriorityMaxValue
 	// We stop inferring and report a circularity if we encounter duplicate recursion identities on both
@@ -374,7 +374,7 @@ func (c *Checker) invokeOnce(n *InferenceState, source *Type, target *Type, acti
 	n.targetStack = n.targetStack[:len(n.targetStack)-1]
 	n.sourceStack = n.sourceStack[:len(n.sourceStack)-1]
 	n.expandingFlags = saveExpandingFlags
-	n.visited[key] = n.inferencePriority
+	n.visited.set(key, n.inferencePriority)
 	n.inferencePriority = min(n.inferencePriority, saveInferencePriority)
 }