Hello ranoano,
If I'm understanding you right, you're seeing flickering when scrolling through your photos in the Photos app on your Mac. Also, this only started happening after updating to macOS Catalina. This doesn't sound like something that is expected to happen after updating your macOS version, so I can understand why you would reach out for assistance. I'm happy to help you.
There are various possibilities as to why this may be happening. Try out the following steps to troubleshoot the issue and narrow things down further. Test out Photos after each step:
- Quit Photos and reopen it: Quit apps on Mac.
- Disconnect any devices connected to your Mac other than the power cable.
- Restart your Mac.
- Reset the NVRAM: Reset NVRAM or PRAM on your Mac.
- Test Photos while booted into safe mode: Use safe mode to isolate issues with your Mac. Test Photos once more after restarting your Mac and logging into your normal user account.
- Test out photos while logged into a brand-new user account: Set up users, guests, and groups on Mac. For this step, you will have to add some photos to the Photos app.
If the issue persists, respond back letting me know if you experienced it while booted in a new user account.
Thanks for using Apple Support Communities.
Take care!