# UNDF: UNDF-2026-000000225
--- a/model/labels/labels_slicelabels.go
+++ b/model/labels/labels_slicelabels.go
@@ -415,14 +415,22 @@ func NewBuilder(base Labels) *Builder {
 // Labels returns the labels from the builder.
 // If no modifications were made, the original labels are returned.
 func (b *Builder) Labels() Labels {
 	if len(b.del) == 0 && len(b.add) == 0 {
 		return b.base
 	}

+	// Build O(1) lookup sets once rather than calling slices.Contains (O(D)) and
+	// contains (O(A)) inside the O(L) base loop — avoids O(L×D) and O(L×A).
+	delSet := make(map[string]struct{}, len(b.del))
+	for _, n := range b.del {
+		delSet[n] = struct{}{}
+	}
+	addSet := make(map[string]struct{}, len(b.add))
+	for _, a := range b.add {
+		addSet[a.Name] = struct{}{}
+	}
+
 	expectedSize := max(len(b.base)+len(b.add)-len(b.del), 1)
 	res := make(Labels, 0, expectedSize)
 	for _, l := range b.base {
-		if slices.Contains(b.del, l.Name) || contains(b.add, l.Name) {
+		if _, inDel := delSet[l.Name]; inDel {
+			continue
+		}
+		if _, inAdd := addSet[l.Name]; inAdd {
 			continue
 		}
 		res = append(res, l)
--- a/model/labels/labels_common.go
+++ b/model/labels/labels_common.go
@@ -204,12 +204,16 @@ func (b *Builder) Set(n, v string) *Builder {
 // Get returns the current value of label n from the builder. It checks the
 // pending-add slice first, then falls back to the base label set.
 func (b *Builder) Get(n string) string {
-	// Del() removes entries from .add but Set() does not remove from .del, so check .add first.
-	for _, a := range b.add {
-		if a.Name == n {
-			return a.Value
-		}
+	// Del() removes entries from .add but Set() does not remove from .del, so
+	// check .add first.  Linear scan over b.add is acceptable: the add slice is
+	// bounded to the number of labels set in a single relabel rule (typically ≤5).
+	// For callers that repeatedly call Get on a large add slice, build a map once.
+	for i := range b.add {
+		if b.add[i].Name == n {
+			return b.add[i].Value
 		}
-	if slices.Contains(b.del, n) {
-		return ""
 	}
+	for _, d := range b.del {
+		if d == n {
+			return ""
+		}
+	}
 	return b.base.Get(n)
 }