QuickTime Player Screen Recording Lags my Computer

1st, think of what you are doing... you are "performing" for an audience (albeit a later audience, a performance nonetheless) - so, you should prepare as if you will be in front a room full of people:

• start up your Mac from OFF - every time
• turn off anything that automatically loads - ( Login Items )
• Launch your performance piece - the Game
• Launch QT and prepare it to 'start recording'

clean start, only apps necessary running

next, 4GB RAM seems to be the "actual" MINIMUM - as opposed to the "2GB minimum REQUIREMENT" for installed RAM. I have seen many here - without hesitation - suggest that a "RECOMMENDED minimum" is 8GB. RAM usage, disk space usage & 'gases in a chamber' - all 'expand to fill the available space.'

You can "watch" what is actually being consumed in system resources with Activity Monitor.app - its many reporting tools and sort filters should let you see exactly what is happening at any given moment

It is "Show Business" - don't 'perform' your show spontaneously.

You might try acethinker free screen recorder. It's a web-based application, so it cost less resource than QuickTime Player. I always use it to record my macbook pro screen, everything works smoothly. Hope it helps you.