Hello ,
i was searching your problem for many months before you , i had the same problem.
Well first of all , Flashlights cause many players to drop fps, just as Dlights from third-party plugins, maps, etc.
if using OpenGL, even with a high-end graphics card, if you go into a map that uses light or light_spot entities that have their "style" set to something else other than default (such as flicker, pulsing, etc.) your fps will be completely wrecked when looking at them. Oddly enough, changing to D3D would make it perfectly fine.
So basically, OpenGL just can't stand any sort of dynamic lights, for some reason.
in other words , no matter how powerful your graphic cards is , running cs on OpenGl will make it odd and too laggy .
Regardless, you could try setting in console or edit in config :
in the console and see if that helps.
i still recommend you to use D3D mod because it's OpenGl internal problem .