Demo recording should have no impact what so ever on your fps. Sorry gotta dispell that rumor. Its a cs urban legend.
Recording and encoding is done at a very simple layer of the engine, simply logging a set of inputs and data streams into a text file which is then compiled in a rudimentary way by the cpu - in terms of run time your looking at less than a fraction of a percent of your cpu. The only time you'd may possibly get a spike would be if the memeroy in use exceeded your free system ram while playing and then it stores a page file.
I'm perma demoing myself, once had a 5 hour long demo which was just over 600meg a few years ago on a very old rig when I fell asleep spectating a public server. Never experienced performance issues. Biggest test cs can throw at your gpu is smoke particles, end of story.