isDroppingFrames method
Check if we're consistently dropping frames (< 50 FPS)
Implementation
bool isDroppingFrames() {
final avg = averageFrameTime;
if (avg == null) return false;
// At 60 FPS, frame time should be ~16.67ms
// At 50 FPS, it's 20ms
return avg.inMilliseconds > 20;
}