I'm running XUbuntu 14.04 and I have a AMD Radeon HD 5450 graphics card and two Dell UltraSharp U2415 24" displays (1920x1200) where one display is rotated to be portrait.
My system has never been super fast. It's a Intel(R) Core(TM) i7 CPU S860 @ 2.53GHz CPU with 32 gigs of RAM but it's been good so far (I'm not a gamer).
The other day I did a reboot because the mouse cursor on one of the displays was scrambled where as on the other it was fine (I never found the cause to this).
After the reboot, I noticed that my display rendering had become super slow. Dragging any window across the screen now results in a severe lag where before it was kinda slowish but not like this.
I started fiddling with my settings and I noticed that when I set my rotated display to "normal" the system is slightly faster. After some Googling I found out that the rotation is done by X11 and I assume this is happening in software.
I have removed and reinstalled drivers (a few times). I have manually sifted through my xorg.conf but nothing seems to really make a difference.
Is it just that my system is to slow or are there any optimisation steps that are available to me?
By following the instructions in the accepted answer for this question I was able to resolve my issues:
How do I remove the proprietary ATI drivers?
After removing the driver I did not reinstall any specific ATI drivers.