I have a business colleague who lives in Phnom Penh and meets me in KL. He always uses Air Asia nowadays and they have always been on time both ways.
The KL base has now converted entirely to brand new A320s, and the older 737s have gone from here.
If you want to get a feel for the punctuality, have a look on the web at the KL airport arrivals board over several days, and compare to MH for equivalent services.