My solution to this has to save, quit, and restart every so often.  This seems to help quite a bit.  As to the fact that we still have memory leaks somewhere, yes I've noticed that.  Fortunately, they are orders of magnitude smaller these days, so you can play much longer between those annoying OOM chime noises...

I usually am able to play for hours between slowdowns, so much much better than the 'old days'.

Autosave is also a savior on this.  If you have an OOM, you can usually load the last autosave or two and continue from where the autosave game left off.

I have an older system, an AMD Phenom II 9600 quad core with 4 GB of RAM ... I've had no problems with lag or unresponsiveness except in the campaign.  At certain points, things ran very slowly ... one of the worst lag-inducers was a tactical combat map in the campaign I don't recall having seen outside of the campaign which had a lot of trees, several in every tile in fact, and each tree had shadows, I think the overwhelming number of trees and shadows are what caused that battle to lag so badly.  I haven't tried the campaign though since the version it was put in (was that 0.90?) so I dunno if its improved.

Same thing for me. It starts out nicely, gradually getting slower, and by end-game I ended up alt-tabbing between the game and other stuff, because every turn was taking so long(!).
Reloading a save game never helped, and it also takes ages to load save games.

Yep I have similar problem, but I agree I can play for 6-8 hours before I see it, I dont really see more of a slowdown as a OOM CTD.

Still much improved from the E:WOM days...