It can be a lot of things....
You can turn down some eye candy to reduce overhead in the graphics rendering.(game-play)
Always record to a drive other than your system drive or separate RAID'd volume.
You can record at lower resolution and re-render to higher resolution in editing.
Lower your memory overhead by killing all non-essential processes.
Keep all auto-updating of apps turned off.
Disable all antivirus software any anti-malware apps with active scanning.
I use Fraps(licensed) and recording at that resolution or better is very taxing on hardware, some games more than others.