To be honest, it's the least quick, and least attractive part of what ultimately is a very pretty journey (if you keep going north to Bangkok!). With customs it can be quite slow. There are loads of 'super coaches' with seats equivilant to airlines first class seats, maybe consider them if you don't want to fly?

Although, don't let me put you off, I always train where possible, but think this section is probably not worth the hassle if you are time constrained.

We used this method about 7 years ago the journey down was a night journey and left KL about 1030PM and arrived at Singapore about 7AM in the morning the journey is not that exciting and you have to book very early to get a sleeper (but the prices for all classes are very cheap, I think we paid £30 for 2 people for first class seats) The station in Singapore is terrible (can someone confirm if this is still the same), weirdly the return journey seemed about 2 hours quicker.
Another method of transport to consider are the deluxe express coaches.