WARNING: Keep in mind that this tweak is intended for people using the "Classic Start Menu" mode to browse trough the Start Menu.
If your Start Menu loads right away when you click on it, but goes slow while you browse trough it, this will certainly solve your problem. It's quite simple actually, just follow these steps:
1. Right Click on your taskbar and choose "Properties"
2. Choose the "Start Menu" Tab and then click on "Customize"(Classic Start Menu :) Obviously)
3. Scroll Down the "Advanced Start Menu Options" list and uncheck the "Use Personalized Menus" option, click "OK", then "Apply" and "OK" to finish.
There's no need to reboot, you'll see the difference right away. Another good advice to keep you start menu working smooth is to restrict the number of programs and icons listed as much as possible, keep in mind that it works just like any other regular folder, the less you index, the faster it gets.