It took me an hour to clear immigration last night at 9pm. There were no special queues for people that already had visas, all booths were in action and in use for any entry type. Each booth is only able to process about 30 people per hour, luckily it was not very busy and there were only 30 people in the queue that I was in. Only pictures were being taken, when they add fingerprinting the delays will no doubt increase. In my experience there can often be over 100 people in each queue... 3 hours or more in peak times?. Can't imagine that VIP would work either. Make sure you pay for a visa BEFORE joining the queue, otherwise its back to the end.
Dave