Immigration at KLIA is a bit unpredictable. I've travelled there a number of times in the last 5 years and sometimes I can be through immigration in no time, and sometimes, it's a long wait.

Also depending on how you plan to get around KL, traffic can be appalling both in the morning and evening rush hours.

I realise someone has already said you are talking about KL, but I would seriously consider other Malaysia locations - JB (Johor Baru) is just over the border to Singapore and it reduces any chances of missed flights; other places worth a visit which may be less busy - Penang and Malacca.