Note, this does not work until you log out of the game and back in.

http://forums.wow-europe.com/thread....05052305&sid=1

Fixing combat log spam when you zone (src)

How to stop framerate lag due to refiltering when you zone:

Issue: When you zone from an instance to the outside world, the combat log goes through a massive loop, pasting every combat log message in the last 5 minutes.

Why would I want to do this fix?
· You have framerate issues when zoning
· You have recently been in a combat sessions where there are many combat log events (e.g. raid boss encounters)

Instructions
1. Select the “Everything” tab
2.
Code:
/script Blizzard_CombatLog_CurrentSettings.settings.showHistory = false
This will prevent all combat log messages from being re-filtered when you zone.

Side-Effects:
This will also prevent you from seeing old messages with the “Everything” tab.

How to Undo:
If you accidentally set this on the wrong tab, select the broken tab and type:
Code:
/script Blizzard_CombatLog_CurrentSettings.settings.showHistory = true
These settings will be saved when you logout.