It seems that this game eats up a lot of ram. I only have 2 gb though. Anyway it has got to a point where saves take more than a minute, and loading the save screen takes around 2. I can see it deteriorating from there as well...
Someone mentioned erasing the achievements content helps. Do you mean erasing the content of the txt, or erasing the actual text file (or even the entire folder?).
I know there's a bug with achievements that makes the save files ultra-mega-huge. Use Notepad++ to open the save files (ordinary Notepad can't handle such large files), find where all the achievements are listed (near the bottom) and delete them all. This means you won't be able to play with achievements, however, though I think it's been fixed in the beta patch now (DL it from Paradox forums

It helps to delete save files from time to time too, since it takes much longer to save the game and also load the saves menu if you've a lot of saves.