macOS Big Sur very laggy, sluggish and slow!

I don't see any of the usual suspects in the report. You do have a couple of online backups running in the background, OneDrive and Backup and Sync. OneDrive was noted as having hight CPU use.

Just so we're all on the same page you do know that online/cloud backup facilities are not compatible with Photos libraries, right. They don't offer the same drive formatting that is needed for the complex ecosystem of permissions and hard links that make up a Photos library.

For my 2017 iMac Big Sur has been the best system I've used with it.

Run Activity Monitor and keep an eye on the CPU section to see what's eating up all of the CPU cycles when you notice performance gets sluggish.

You might remove all items from the System/Users & Groups/Login Items preference pane and see if there's any improvement. If there is then add them back one at a time and see if you can narrow down the culprit.

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.

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.

Copy the report

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

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