if you have a visa you still need to go through the immigration process, the two desks for people with visas are right next to the two for people getting visas on arrival. if there are queues they are side by side. It does take less time for the Immigration Officer to process someone with a visa as they do not need to fill out and affix the sticker in the passport (the rest of the process is the same), also the majority of people seem to get visas on arrival so that queue is usually bigger. However there have been times I've been through when the queue for people with visas has been longer than the queue for people getting one. The last two times I have been through, getting a visa on arrival, the first time there was just one couple in front of me and the second there was no-one at all, so i was straight through, while both times there was a double figure queue for the 'with visa' desk.
If there are big queues check out the other set of Immigration desks (there are two sets linked by a corridor at the back of the hall near the tables where people fill out the forms)