There's no arguing this one. The Old Kai is our introduction to fusing via the Potara Earrings, and he's just puny. You could chalk that up to his age, but Master Roshi is no slouch as a fighter despite his many years. Sure, the Old Kai has strong
mystical knowledge, but in a fight, he's pathetic. The Saiyans even initially assumed he must be super powerful, so they tested him by attempting to attack him. All that did was anger the Old Kai, though, since his feeble appearance was no ruse.
This fusion is a perfect example of why the Z Fighters don't use fusion carelessly. When Goku was desperate to fuse with someone against Buu, he contemplated fusing with Dende and Hercule, but this probably would have just decreased his power. The original Kai and the witch who fused with him likely weren't exactly powerhouses individually, but this accidental fusion was just bad for both of them.