I'm sure this is a duplicate, but I couldn't find a ticket for it...
So, if you are a Blufor soldier with an MX rifle, loading the mission will take substantially longer than if you are Opfor. This is not the case if you are playing Opfor and there is Blufor soldiers in the mission, only when you ARE a Blufor soldier.
It has been suggested on the forums that this behavior has to do with the MX rifles/ammo.