If you're getting leg kicks out of it, then it's almost certainly because the bumper or trigger isn't pressed yet. None of the bumper strikes can result in basic leg kicks.
I doubt input lag is the issue from your hardware, but you might be pressing the buttons at very similar times to each other but in the wrong order. That's my best guess from your description.
I really wish I could get the input history HUD into practice mode, but alas, it proved too tricky for the time I had.