I find it almost as quick to load a grease gun from a tub as I do with a cartiridge.
1. Take top off grease tub
2. Take top off grease gun
3. Invert grease gun over pot
4. Simultaneously push down on the barrel of the gun and pull up on the chain/rod to withdraw the piston
5. Lock chain/rod in position when it's full
6. Put top back on grease gun
7. Put top back on grease pot
Of course, this technique does depend on your grease pot having one of those plastic inner covers with a hole in the middle.
