Q: How do i stop my quicktime from lagging when i am recording my games?

You can't do much beyond toggling between QuickTime's "quality" settings to see if you get better results with one setting than another. Graphic-heavy games consume considerable system resources. QuickTime screen recording also consumes considerable system resources. When you have both competing for finite computing power, one or both will inevitably lag.

You might be able to improve performance by maxing out the capabilities of your iMac. The first step is generally to add as much RAM as the machine will handle. The next step is to swap the traditional hard drive for an SSD. The step beyond that: a newer, faster Mac.

One way to help understand what resources your Mac is using at any given time is to employ Activity Monitor

How to use Activity Monitor - Apple Support

• Use your Mac as you would normally up until you are ready to start recording a game session
• Switch to the Finder
• Open Activity Monitor
• Note CPU usage*
• Note Memory usage
• Note how much of each is 'Available'
• Switch to Game Recording methods you normally use & do so

*What you want is to see the graph of CPU usage during a period of Game Recording

You can also see CPU usage in a separate window or in the Dock:

• To open a window showing current processor activity, choose Window > CPU Usage. To show a graph of this information in your Dock, choose View > Dock Icon > Show CPU Usage.
• To open a window showing recent processor activity, choose Window > CPU History. To show a graph of this information in your Dock, choose View > Dock Icon > Show CPU History.

One thing most folks forget is to Quit everything out BUT the Game and QT - you can recover a lot of resources for their use

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