As been mentioned again and again, and again... this has to do with a bug related to memory. LW eats up a whole lot of RAM (up to 90 MB), and when it starts eating up your RAM, it naturally slows down. So you either
1. allow LW access to more RAM, or
2. if you haven't got enough RAM (128 MB is not enough!), buy more, or
3. keep complaining to the programmers so that they'll start doing something about it.

It'll help if you read some of the previous threads before you add one. Usually, you'll get some sort of answers. And it helps the rest of us from getting yet another new thread that moans about something we've already done before.