some junk is magma safe and then you actually made the problem worse because now the game is tracking the temperature. While the non-magma safe stuff burns you'll experience more lag too. It's why I usually use atom smashing. Killing 100+ cats should make a difference, however...
selling a lot of stuff helps, but if you take it in perspective it may be nothing. Why? Well, selling 1000 pieces of clothing while you also have 35.000 bolts and 55.000 prepared meals and what not else is not going to make much of a difference. Similarly, killing 100 cats may help but if you still have 100+ dogs, 100+ turkeys and 100+ of another dozen animals, the difference is hardly noticable
Lastly, items may be still in a buffer somewhere. Quit and restart the game and a difference may be seen. I once managed to go from 20 fps to 50 just by disposing of items and creatures, though that was on a bad computer and already a small fort
Well my fort has pretty much become self sufficient and has a surplus of candy, so there hasn't been a need to really import anything. So I've ended up with a huge amount of food and goods tucked away, along with my 'emergency food supply' room filled with about 20 pigs.
Would it improve things to just dump everything non-vital onto the traders and start from 'scratch', or is FPS death unavoidable once a certain amount of items has been reached?
Oh! And there was also an accident with the river draining, so it's draining down 3 Z levels into a huge cavern and draining off-site. Would that impact FPS as well?