Not sure this might be of use to anyone but I changed my ATI HD7770 gfx card for an almost equivalent but newer model R7 260X. I installed newer drivers as my old Catalyst v13.4 drivers didn't support the new card and while WEI game me 0.1 extra Gaming Graphics score (from 7.6 to 7.7) in ET my fps went from almost constant 125fps before to ~20-30fps and horrible lag/blur
This evening you might have seen me on/off the server a number of times trying different versions of Catalyst's and settings on them but none seemed to help and using v13.4 wasn't an option with this fairly new model of card. I suspect ATI introduced or dropped something after v13.4 of their Catalyst drivers that effectively kills FPS in older OGL games like ET as also tried upgrading the drivers with my old HD7770 card months ago and experienced exactly the same issue but luckily downgrading the driver back to v13.4 was an option.
Anyway, it seems after much searching one fix is an ET cvar
seta r_primitives 2 magically bought back constant 125fps. I've no idea what it does but hope this might help anyone else who upgrades their graphics and/or Drivers and suddenly gets stupidly low fps
For Info, my PC is overkill for ET with a i7-3370, 16GB ram, ATI R7 260X and I run ET @1920x1080 so there should have been no excuse for it not running a 10+ year old game at this resolution other than maybe ATI not giving a toss about old (but great) games with their latest drivers and dropping proper support for whatever version of OGL ET uses.
Long live ET