And in order to hit the target you need to aim, not just reload. Aiming takes time , which is why there is a difference in theoretical rate of fire and actual rate of fire.
That makes it even worse though. Both tanks have to aim, both tanks fire, both tanks miss. But here's the kicker, the Panther stays trained on the IS-2 while it reloads. The IS-2 has to level the gun, reload, then aim again. Not only does the Panther reload faster, but it may also make adjustments to it's aim while the IS-2 reloads slower and has to aim every shot as if it were the first.
Why not make the ISU an heavy tank hunter like the elephant? Completely disregarding all of it's AI capabilities.
Because that doesn't make sense, a 152mm Howitzer firing HE Shells, but it can't hurt Infantry? That's like having the short-barrel StuG III E take the SU-85's job.