Before you go reinstalling the old Mac system, have you explored the basics
a. What are the properties of the source media? Are you working with video recorded with variable frame rate of are you dealing with DVD video files (vob)? What is this speed to which you refer - program recovery between each Timeline action, slow program response in general, other?
b. What is your project preset to match the properties of the source media? If you do not know, please tell us the readings for Editing Mode, Timebase, Frame Size, Pixel Aspect Ratio (even if they look grayed out) under Edit Menu/Project Settings/General.
Did you download and install the 12.1 Update using an opened project's Help Menu/Updates?
Video card/graphics card driver version up to date according to the manufacturer of the card?
Problem exists with and without antivirus and firewall(s) disabled?
Is your idea to go back to the "old system" based on "it worked before but not now"?
Please review and consider and then we can discuss what next.