Recognize pointer down event without delay in Flutter (iOS, Android) - user-interface

Question

T'd like to track pointer events in Flutter (pointer down/up and pointer position) but there's always a delay (about 500ms) between the moment a finger touch the screen and the PointerDown event, unless I move my finger immediately.
The standard GestureDetector doesn't handle that (there is a delay to recognize TapDown event), so I tried the code below but to no avail: the delay is still there.
Any idea how could I solve this problem?
import 'package:flutter/foundation.dart';
import 'package:flutter/gestures.dart';
typedef PositionEventHandler = void Function(Offset);
class MySimplePointerGestureRecognizer extends OneSequenceGestureRecognizer {
final PositionEventHandler onPointerDown;
final PositionEventHandler onPointerUpdate;
final PositionEventHandler onPointerUp;
MySimplePointerGestureRecognizer(
{#required this.onPointerUp,
#required this.onPointerUpdate,
#required this.onPointerDown});
#override
void addPointer(PointerEvent event) {
startTrackingPointer(event.pointer);
resolve(GestureDisposition.accepted);
}
#override
void handleEvent(PointerEvent event) {
if(event is PointerDownEvent) {
onPointerDown(event.position);
}
if (event is PointerMoveEvent) {
onPointerUpdate(event.position);
}
if (event is PointerUpEvent) {
onPointerUp(event.position);
stopTrackingPointer(event.pointer);
}
}
#override
void didStopTrackingLastPointer(int pointer) {}
#override
String get debugDescription => 'simplePointerTracker';
}