diff --git a/traversal.go b/traversal.go
index c495eea..09b0b66 100644
--- a/traversal.go
+++ b/traversal.go
@@ -657,6 +657,20 @@ func getChildrenWithSiblingType(parent *html.Node, st siblingType, skipNode *htm
 		}
 	}
 
+	// For the collect-all cases, count the results first so result can be
+	// presized, avoiding append's repeated reallocations.
+	switch st {
+	case siblingAll, siblingAllIncludingNonElements, siblingPrevAll, siblingNextAll:
+		n := 0
+		for c := iter(nil); c != nil; c = iter(c) {
+			n++
+		}
+		if n == 0 {
+			return nil
+		}
+		result = make([]*html.Node, 0, n)
+	}
+
 	for c := iter(nil); c != nil; c = iter(c) {
 		// If this is an ...Until case, test before append (returns true
 		// if the until condition is reached)