Denouncing shouldn’t add lag. On the first closure detection you can send the key down code. You then need some debounce logic to decide when to send the key up code, but after the key is solidly down you are again in a state to send the key up code as soon as the up begins.
The only time there should be lag is when a very short keypress happens, the key up might be delayed while the controller rules out bounce.
I don't claim that 15ms of lag is to be considered good, but the problem that one has to solve is debouncing the switches.