With the last Ubuntu Lucid 10.04 come a new graphical module: KMS or Kernel-based Mode-Setting.
If you have performance problems in some games like Quake 3 or TCE / Enemy Territory, try to disable KMS.
The problem can also be with the mouse pointer (slowness, lag, delay or inacuracy).
Grub 2
Add nomodeset in /etc/default/grub then issue the command update-grub.
vi /etc/default/grub GRUB_CMDLINE_LINUX="nomodeset" update-grub
Grub 1
Add nomodeset at the end of the # kopt line, then issue command update-grub.
vi /boot/grub/menu.lst # kopt=root=/dev/sda1 ro nomodeset update-grub
Someone also told me that disabling KMS solved his Firefox javascript speed problem in Yahoo Mail.