cs 1.6 fps drops while recording demo

i didnt found a thread like this so lets get started.
when i play cs 1.6 thru steam its playable (90-100,80-70 in some places),but when i start recording a demo my fps drops from 100 to 30-60..i think its not my pc problem because when i play nonsteam cs (making tj demos)my fps is 100 const and while recording 100...i dont get it.Why should steam lower my frame rates????