mp4 always 20 FPS video gameplay recording also lag

[vase070]

hi i have some problem so when i run the game it runs 100 fps NO LAG OR ANYthing when i start recording it starts lagging fps drops to 40-50 maybe 30 sometimes and when im done recording video framarate: 20.000 fps ..... i set the program to record 60 fps im pretty sure my pc can handle recording cs 1.6
when i use AVI it runs PERFECTLY but the thing is 1 min recording 1.2 gb file ...
i am doing something wrong with mp4 ? do i need to download a codec or something ? anyone had any exp with this kind of problem
these are my settings i use