Seems like the number of frames per round and the ease of dismembering your opponent change a lot from one room to the next. Is there a pattern to know, or is it just random from room to room?

Does it have to do with the mod of the room?

It has to do with the mod being played in that room. Certain mods have specific settings that you can see and change (if you're an operator for the server) by pressing Ctrl+G.

Another thing
The lower the dismemberment rate
The easier it is to dismember something
Same goes for fractures