Big Sur + Screen Lagging

At this point I would suggest that you run an EtreCheck report, and optionally, post it here if you need assistance with interpreting its results. EtreCheck is a diagnostics tool that greatly assists with troubleshooting your Mac ... especially remotely since we can't "see" it.

Ref: Using EtreCheck to Troubleshoot Potential Mac Issues - Tesserax ASC

Thanks for the report. Just to let you know, enabling "Full Drive Access" (as per my user tip) would provide us with access to any kernel panic or app crash reports. Again, these would come in very handy in diagnosing any potential culprits that may be the cause of the screen lags. I suggest re-running the report with this enabled, but that's totally up to you. In the mean time, here are my observations/recommendations based on the report you provided:

• Hardware Information - 8 GB RAM: Unfortunately, 8 GBs of RAM is not going to provide you with optimal overall performance, especially if you run apps concurrently. Also, unfortunately, you cannot upgrade it beyond this. Note that Google Chrome is a memory intensive app and you may want to switch to something like Brave instead.
• [Loaded] uTorrent Web (BitTorrent, Inc - installed 2021-05-12): Using a Torrent app open your Mac to potential to acquire malware/adware. Just making you aware. It's totally up to you if you want to continue to using it.
• Swap Used: 2.20 GB: This is an indication that your notebook has too little RAM. Apps are being swapped out to the system drive which will slow down overall performance.
• Diagnostics Information: Two system processes (syspolicyd & signpost_reporter) are causing excessive CPU usage. This will appear to slow down your Mac.