Screen recording at 60 fps

I'm interested in trying to record my screen so I can do walkthroughs of games and such. Usually these are action games, so high resolution and fps are both important.

I'm using kazam to try and record my screen at 1920x1080x60 fps.

However, the video result lags quite a bit. I would estimate the result is maybe 5-10 fps. I did a test by recording desktop to ensure the issue was the recording, not the game.

I had 2 gigs of memory free and the load never went above 3.0. iostat showed iowait at 0.39%. The resulting test clips are usually 10 megs or so, and I'm fairly certain that Kazam compresses on the fly and stores the videos in memory until it asks you to save them to disk, so I would imagine io shouldn't be an issue.

So if the limiting factor isn't cpu, memory, or disk io, what's going on here?

Thanks!