Mobility

I believe that only works for buffs. Because I can totally use a potion while I'm using triple throw. And gunslingers can totally back me up when I say that recoil shot doesn't cause this lag at all.

If anything, compare it with teleport, I guess, but longer time without a potion. Heck, you can't even throw stuff from your inventory in midair.

Apparently, there's something in the coding that requires your MP (and possibly position) to be updated/synched after you use a buff/movement skill, before you can use a pot. This does not happen with attacks, hence why recoil shot works fine all the time.
It's very easy to see when you lag a lot. back in the days of lag spikes and when I played a priest, I could easily see it, my MP would go down about 2 seconds after I tele'd and at that exact moment I could tele again.