That's probably because of the large amount of dx items being rendered on your screen. It happens to me aswell whilst viewing the scoreboard in a full server. My framerate takes a dive when I view the scoreboard or use something like the dxspeedometer, hence me inferring the dx rendering. You sure you've got a good computer?
well these are my specs
CPU:quad Core i5 2.7 ghz ( 4 cores )
Ram:4GB
HDD:500 GB
GPU:Geforce 8800 GT 512 mb
What can u say about an old game like GTA even in GTA:IV igot 68 fps maxed out