Don't create a new iterator for every time step.

Signed-off-by: Charles Korn <charles.korn@grafana.com>

promql/engine.go

@@ -1830,6 +1830,7 @@ func (ev *evaluator) evalTimestampFunctionOverVectorSelector(vs *parser.VectorSe
 	if err != nil {
 		ev.error(errWithWarnings{fmt.Errorf("expanding series: %w", err), ws})
 	}
+	it := storage.NewMemoizedEmptyIterator(durationMilliseconds(ev.lookbackDelta))
 
 	return ev.rangeEval(nil, func(v []parser.Value, _ [][]EvalSeriesHelper, enh *EvalNodeHelper) (Vector, storage.Warnings) {
 		if vs.Timestamp != nil {
@@ -1838,7 +1839,6 @@ func (ev *evaluator) evalTimestampFunctionOverVectorSelector(vs *parser.VectorSe
 			vs.Offset = time.Duration(enh.Ts-*vs.Timestamp) * time.Millisecond
 		}
 		vec := make(Vector, 0, len(vs.Series))
-		it := storage.NewMemoizedEmptyIterator(durationMilliseconds(ev.lookbackDelta))
 		var chkIter chunkenc.Iterator
 		for _, s := range vs.Series {
 			chkIter = s.Iterator(chkIter)