Thousand apology Veloo. I didn't mean any harm in here. I was hoping that whatever equipments you have there must be of the latest or new one. However, I'm a guy frequently fly to the big US cities .ie, JFK, O'Hare, DFW, Atlanta, LAX, SFO, etc...Their traffic condition is a nightmare. Of course they have more than one runway. Never the less, they can coup up with the demand day by day. Most delay is caused by security problem and weather (storm, heavy snow, etc..) Improvement can be done, ie. reduce separation, multiple approach sectors/controllers, sim-ops approach/departure, to name a few. I understand you have a single runway at this moment. May be after the work completed, the situation will be better.
Ok, what about asking flight to descend to an intermediate level before VKN from SIN ATC, ie. FL 290/280. and then KUL sector take over at/after VKN onwards. It's no point hanging at FL 370/390/400 while passing VKN, I'm sure that position is pass over your descend point for that kind of level. It's better to start your descend slightly earlier to intermediate level than spending extra minutes in high level cruise and charge the plane like hell after passing VKN. Worst if there will be speed control after that. Passing VKN at FL290/280 within KUL sector will be just nice for the next descend clearance. And keep the departing plane to cross VKN at FL300 or above so as not to disrupts other traffic that inbound for KUL/SZB.
I gave my 2 sen worth of suggestion/s in here. It's just a suggestion, if it doesn't suit your needs, than try something else. As for Air Asia never have any complaint, I could not comment on that. Perhaps, they don't fly to Primary airport beside than KUL. It's always secondary airport that they like to go. They haven't seen major airport like the above I have mentioned or even some major airports in the rest of the world. So, they can't make much comparison until they have experienced the difference between one major airport with another.
Cheers.