Yeah... actually they should have added an option not to have memories at all. It doesn't add anything new or different to the game and I personally find them really unuseful and annoying, and it obviously will cause some game lagging after a few simweeks. Before installing Generations my game used to load really fast, no longer than 5 minutes... now, if it takes 15 minutes to load I find myself lucky. It's usually taking 20 minutes to load... and I'm playing a new file, only 3 simweeks old. Don't want to think how long it will take when I'm in week 10 or so.
Anyway... changing neighborhoods would delete memories, so maybe that's an option to avoid game lagging from excess of townies' memories.