You would have to explain what you mean by 'displayed separately' and 'fps'. Some people see FPS and lag or memory/cpu utilization as all one in the same for 'performance'.

If you mean the vanilla 'general' and 'combat log' chat windows displaying side by side and 'combat log' displaying 'everything'. I would be doubtful of 'significant; reduction in frame rates.This information is obviously low priority buffered text that is inbound on the game nomatter what for the most part, to the unit frames, combat tools, a whole host of crap in the titan/fubar/broker world, quest logs, cooldown junk, autobar achievements, etc.

If you mean 'combat log' to be parrot or similar where a setup might be rigged to dynamically fade rainbow colored shadowed bordered text with spell icons and so forth, sure. I could believe that.

But these are systems where 1024x768 and 24bit in major cities is a huge issue, you see a big aoe fest and your walking into a wall and so forth. And zone loading is quite long.

I would say again that the minimap tracking is at times and for certain classes a 'significant' resource hog.