Once you remove one of the o-rings and grease the plug its not bad. At least, not any slower than any other stock pistol where you have to remove the feed plug to load. Compared to a top-loaded Zeus? Slow, but probably faster than any other overhead-springfed pistol. I'm just spoiled after having used a topfed Zeus is all.
One of the tricks to a fast load with the SA17 is to stick a loaded tube into the loading port BEFORE you run out of ammo. They press in and stay there well enough to run around with for a little bit. That way you run dry and its already there, do the rest of the loading procedure (pull back spring follower, turn plug) and the paint just drops in. Twist the plug back to the feed position, tap the spring tab to release it, and then yank the now-empty tube out and toss it in your pocket, dump pouch, whatever.