Photos going really slow on Big Sur and crashing.

There is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This user tip describes what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community   

That being said you've installed what most of the experienced contributors to these forums consider to be akin to malware: CleanMyMac. It's been known time and again to cause more harm that good to Macs. Uninstall according to the developer's instructions.

Also uninstall Memory Cleaner. It works agains the system's own memory management.

Gemini and Gemini 2 are also not good to use. Used on Photos libraries they've damaged libraries. Also used on hard drive with systems they could potentially mess them up. You just don't need them. Rebooting frequently clears out the temporary system cache and swap files and an occasional Safe Mode boot will help by clearing out temp files and running repair routines on the drive. You should also remove them.

You can check to see if you've removed all of the supporting files by downloading and running Find Any File to search for any files with the application's name  and the developer's name in the file name.  For the above mentioned software you'd do the following search(es): 

1 - Name contains cleanmymac

2 - Name contains macpaw

3 - Name contains memorycleaner

4 - Name contains gemini

Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.  

You have a number of external hard drives, Seagate and Western Digital. They both come with software for managing the drives but are unneeded. Did you install that software? If so it should be removed according to the developer's instructions as they only interfere with the systems disk management routines.

Where is your library located? If on an external HD how is the drive formatted and connect4ed to your Mac?

How much free space do you have on your boot drive?

How much RAM do you have?

Download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Additional Text button to include the report in your reply.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Then we can examine the report and see if we can determine what might be causing the problem.