If I install Catalina on my MacBook Pro Retina 15" (mid 2012), will iMovie work better?

Whether iMovie works better on Catalina vs. Mojave is a matter of opinion. I run Mojave and iMovie runs great with no problems. I haven’t used Catalina so can’t compare from personal experience. From the posts on this forum it appears that iMovie doesn’t run any better on Catalina. Catalina is a 64 bit operating system whereas Mojave is 32 bit. Therefore your 32 bit applications won’t run on Catalina. Also certain legacy media won’t play on it. Upgrading to Catalina will automatically upgrade your Photos app to Photos 5. Photos 5 substantially reoganized Photos 4. There have been many frustrating issues reported re Photos 5.

So, the above considerations would be pertinent re your decision as to whether or not to upgrade. If you upgrade you first should convert to a modern format whatever legacy media that you may have on your Mac.

Perhaps someone will chime in here who has used both operating systems with iMovie.

— Rich