IF you are using a Radeon GPU on Lucid, try disabling Kernel Mode Setting (see https://wiki.ubuntu.com/X/KernelModeSetting ). According to bug reports, there are 2D&3D performance problems on (some) Radeon cards with KMS enabled. Personally, disabling KMS solved the problems with Youtube on my P4 2.4 Ghz with Radeon 9600.
Do the following:
Create a module configuration file with the following command:
Copy the following line to the file:
sudo nano /etc/modprobe.d/radeon-kms.conf
Save the file and restart the computer.
options radeon modeset=0
The bug reports referenced:
https://bugs.launchpad.net/ubuntu/+s...ox/+bug/562293 (See comment #2 and forwards)
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=583653 (Lucid has backported KMS-related Xorg stuff from kernel 2.6.33, but anyway...)
https://bugzilla.redhat.com/show_bug.cgi?id=557420 (not the same kernel version as in Ubuntu Lucid, but anyway...)