Put your belts on guys, because here comes some thread veer.
444, to be honest the AK has many disadvantages for use in 3 gun. That is why amongst the serious competitors the AR is by far the dominate gun. I shoot a Vepr or a FAL because ARs don't suit me. (doesn't mean I don't know how to use them though).
AK mag changes are slower by nature, but with lots of practice a sub 1 second mag change can be performed. (not from an empty chamber, having to manipulate the bolt will add at least another .5 on there). I'm talking lots of practice though. I shoot with my support hand holding the magazine like it is a vertical fore grip. As soon as I want to reload I hit the release with my thumb, pivot the gun forward, and let go of the mag as I reach for the new one. The rock and lock doesn't slow you down to much if you have practiced exactly where to put that new mag. Plus it is always fun to watch guys slam AR magazines in, only to have them fall out and hit them in the foot right after.
(practice guys!)
The lack of bolt hold open kills you if you run compleatly dry. Try not to do that.
I can manipulate an AK safety with my trigger finger, so it is a draw for me. Most folks this is going to be much slower as it will require them to remove their hand from a firing grip, or use their support hand and reach around the gun.
AK sights suck. If you are shooting limited (iron sights) then the AR sights are going to be better for any sort of precision work. However the AK sights are faster at closer ranges. Good work can be done with practice with either, and accurate hits can be made out to 30 yards or so just by using the front sight assembly as an aim point. If you shoot with optics then it is a moot point, however the AR is much easier to mount optics on. I shoot irons mostly so I installed some Krebs AR style sights on my Vepr.
The AR has far better recoil characteristics. Recoil is straight back and mild, the AK has kind of a floppy/bouncy feel. I'm one of the better AK shooters that I'm aware of and even then I can conistently reduce my split times when I shoot an AR. The difference really isnt' that great unless you are really into hitting targets multiple times and fast. I doubt most people would really even notice unless they were being timed.
So why do I use the slower, more un-ergonomic weapon?
I just like it better, and it is kind of fun to be the guy shooting something different. I can get practice ammo for around $80 a case. I really like shooting an FAL in 3 gun, however ammo costs about twice as much so I don't get the trigger time that I really like. Heck, I don't get the trigger time that I like with the cheap ammo either.
So even with lots of practice the edge is going to go to the AR for flat out high speed competition.
Of course like anything else, your mileage may vary. I'm currently building a pump shotgun race gun, (like I said, I like being different) and after that I think I'm going to build an AK based race gun. 5.45 Vepr, heavy duty muzzle brake, EO Tech holographic sight, slicked up everything. That should be fun.
All I need is $ of course.