@@ -21953,8 +21953,12 @@ func (c *Checker) getUnionOrIntersectionProperty(t *Type, name string, skipObjec
 // unionOrIntersectionPropertySources describes the constituent properties from
 // which a union or intersection property would be synthesized.
 type unionOrIntersectionPropertySources struct {
-	singleProp           *ast.Symbol
-	propSet              collections.OrderedSet[*ast.Symbol]
+	singleProp *ast.Symbol
+	// props holds distinct constituent properties in first-seen order, or is
+	// empty when every constituent shares singleProp. propsByKey indexes it
+	// only once it is large enough for hashing to beat a linear scan.
+	props                []*ast.Symbol
+	propsByKey           map[*ast.Symbol]struct{}
 	indexTypes           []*Type
 	propFlags            ast.SymbolFlags
 	optionalFlag         ast.SymbolFlags
@@ -21966,7 +21970,30 @@ type unionOrIntersectionPropertySources struct {
 // isSynthetic reports whether the sources require a new synthetic symbol rather
 // than reusing (or cloning) singleProp.
 func (s *unionOrIntersectionPropertySources) isSynthetic() bool {
-	return s.propSet.Size() != 0 || s.checkFlags&ast.CheckFlagsReadPartial != 0 || len(s.indexTypes) != 0
+	return len(s.props) != 0 || s.checkFlags&ast.CheckFlagsReadPartial != 0 || len(s.indexTypes) != 0
+}
+
+// addProp adds prop to props if it is not already present, preserving
+// first-insertion order.
+func (s *unionOrIntersectionPropertySources) addProp(prop *ast.Symbol) {
+	if s.propsByKey == nil {
+		if slices.Contains(s.props, prop) {
+			return
+		}
+		s.props = append(s.props, prop)
+		if len(s.props) > orderedSetMapThreshold {
+			s.propsByKey = make(map[*ast.Symbol]struct{}, len(s.props))
+			for _, p := range s.props {
+				s.propsByKey[p] = struct{}{}
+			}
+		}
+		return
+	}
+	if _, ok := s.propsByKey[prop]; ok {
+		return
+	}
+	s.props = append(s.props, prop)
+	s.propsByKey[prop] = struct{}{}
 }
 
 func (c *Checker) getUnionOrIntersectionPropertySources(containingType *Type, name string, skipObjectFunctionPropertyAugment bool, sources *unionOrIntersectionPropertySources) bool {
@@ -21976,7 +22003,6 @@ func (c *Checker) getUnionOrIntersectionPropertySources(containingType *Type, na
 	var checkFlags ast.CheckFlags
 	var optionalFlag ast.SymbolFlags
 	var singleProp *ast.Symbol
-	propSet := &sources.propSet
 	var indexTypes []*Type
 	if !isUnion {
 		checkFlags = ast.CheckFlagsReadonly
@@ -22013,10 +22039,10 @@ func (c *Checker) getUnionOrIntersectionPropertySources(containingType *Type, na
 						// back and not `Array<string>.length` when we're looking at a `.length` access on a `string[] | number[]`
 						mergedInstantiations = singleProp.Parent() != nil && len(c.getLocalTypeParametersOfClassOrInterfaceOrTypeAlias(singleProp.Parent())) != 0
 					} else {
-						if propSet.Size() == 0 {
-							propSet.Add(singleProp)
+						if len(sources.props) == 0 {
+							sources.addProp(singleProp)
 						}
-						propSet.Add(prop)
+						sources.addProp(prop)
 					}
 					// classes created by mixins are represented as intersections
 					// and overriding a property in a derived class redefines it completely at runtime
@@ -22082,9 +22108,9 @@ func (c *Checker) getUnionOrIntersectionPropertySources(containingType *Type, na
 		return false
 	}
 	if isUnion &&
-		(propSet.Size() != 0 || checkFlags&ast.CheckFlagsPartial != 0) &&
+		(len(sources.props) != 0 || checkFlags&ast.CheckFlagsPartial != 0) &&
 		checkFlags&(ast.CheckFlagsContainsPrivate|ast.CheckFlagsContainsProtected|ast.CheckFlagsContainsWritePrivate|ast.CheckFlagsContainsWriteProtected) != 0 &&
-		!(propSet.Size() != 0 && c.hasCommonDeclaration(propSet)) {
+		!(len(sources.props) != 0 && c.hasCommonDeclaration(sources.props)) {
 		// A property in a union has a private or protected declaration in one constituent, but is missing
 		// or has a different declaration in another constituent. If the private or protected declaration is
 		// for reading, we don't create a property.
@@ -22115,7 +22141,6 @@ func (c *Checker) createUnionOrIntersectionProperty(containingType *Type, name s
 		return nil
 	}
 	singleProp := sources.singleProp
-	propSet := &sources.propSet
 	indexTypes := sources.indexTypes
 	propFlags := sources.propFlags
 	optionalFlag := sources.optionalFlag
@@ -22146,8 +22171,8 @@ func (c *Checker) createUnionOrIntersectionProperty(containingType *Type, name s
 		links.setWriteType(c, c.getWriteTypeOfSymbol(singleProp))
 		return clone
 	}
-	if propSet.Size() == 0 {
-		propSet.Add(singleProp)
+	if len(sources.props) == 0 {
+		sources.props = []*ast.Symbol{singleProp}
 	}
 	var declarations []*ast.Node
 	var firstType *Type
@@ -22156,7 +22181,7 @@ func (c *Checker) createUnionOrIntersectionProperty(containingType *Type, name s
 	var writeTypes []*Type
 	var firstValueDeclaration *ast.Node
 	var hasNonUniformValueDeclaration bool
-	for prop := range propSet.Values() {
+	for _, prop := range sources.props {
 		if firstValueDeclaration == nil {
 			firstValueDeclaration = prop.ValueDeclaration()
 		} else if prop.ValueDeclaration() != nil && prop.ValueDeclaration() != firstValueDeclaration {
@@ -22240,9 +22265,9 @@ func isPrototypeProperty(symbol *ast.Symbol) bool {
 	return symbol.Flags()&ast.SymbolFlagsMethod != 0 || symbol.CheckFlags()&ast.CheckFlagsSyntheticMethod != 0
 }
 
-func (c *Checker) hasCommonDeclaration(symbols *collections.OrderedSet[*ast.Symbol]) bool {
+func (c *Checker) hasCommonDeclaration(symbols []*ast.Symbol) bool {
 	var commonDeclarations collections.Set[*ast.Node]
-	for symbol := range symbols.Values() {
+	for _, symbol := range symbols {
 		if len(symbol.Declarations()) == 0 {
 			return false
 		}
@@ -22459,15 +22484,15 @@ func (c *Checker) intersectionPropertyReducesToNever(t *Type, name string) bool
 		prop := c.getUnionOrIntersectionProperty(t, name, true /*skipObjectFunctionPropertyAugment*/)
 		return prop != nil && c.isNeverReducedProperty(prop)
 	}
-	if sources.propSet.Size() == 0 {
-		sources.propSet.Add(sources.singleProp)
+	if len(sources.props) == 0 {
+		sources.props = []*ast.Symbol{sources.singleProp}
 	}
 	checkFlags := sources.checkFlags
 	var firstType *Type
 	var firstValueDeclaration *ast.Node
 	hasNonUniformValueDeclaration := false
-	propTypes := make([]*Type, 0, sources.propSet.Size()+len(sources.indexTypes))
-	for prop := range sources.propSet.Values() {
+	propTypes := make([]*Type, 0, len(sources.props)+len(sources.indexTypes))
+	for _, prop := range sources.props {
 		if firstValueDeclaration == nil {
 			firstValueDeclaration = prop.ValueDeclaration()
 		} else if prop.ValueDeclaration() != nil && prop.ValueDeclaration() != firstValueDeclaration {