To make a long story short, After several tests on both my old computer and my friend's brand new powerhorse gaming machine(I7 965 Intel Processor)., I am running 25 man raids at 20 FPS during combats on my low end PC.

my PC :Pentium D 830. 2GB RAM 7800 GT. Old stuff.

Settings are all set to low except Spell details, which is set to medium.

Based on the following post from WOW Europe I created 2 macros and set it on my action bar.

http://forums.wow-europe.com/thread.html;jsessionid=572FE37523ECE081B1614D3BD6531D8A.app06_08?topicId=5103073570&sid=1

Disable combat log
/run COMBATLOG:UnregisterEvent("COMBAT_LOG_EVENT")

Enable Combat Log
/run COMBATLOG:RegisterEvent("COMBAT_LOG_EVENT")

If you switch to your combat log window and hit the "see everything" tab. you will see a lot of stuff scrolling with all the action going around you. Click on the Disable combat log macro and all will stop.

This have no effects on FPS if you are not fighting , at least for me, While preparing to engage the boss I normally get around 65 to 80 Fps . with both combat log enable or disable.

While engaging the boss If I have combat log enable, I jump from those 60s to 1 or 2 FPS.

By clicking the disable combat log macro, I go back to 20s FPS, lovely and sexy as it used to be.

Conclusion. This is not a hardware issue, it is a software problem in the client or in server that is causing FPS drops to unplayable conditions.

These are my 2 cents

Addons and UI items only get to have so much of a processor before it starts bumping into everything else. I've added the information on this thread to the System Performance Guide many months ago so it's a bit surprising to see it bumped back up.