BigSur high CPU use. WindowServer and Kernel_task always on top.

Welcome, Straycat123, to Apple Support Communities!

Since I don’t see any other processes using much CPU, besides the Window Server (which handles Graphical User Interface [GUI] tasks for your Apps), it’s hard to see why your «MacBookPro13,1 (2016, i7 2,4Ghz, 16GB ram) runs slow»., or even why the Window Server is using «66% of CPU going to 80%, even more sometimes.»

The typical issue with slow response, high CPU usage for Big Sur, relative to Catalina, is old, incompatible software: including background tasks for such software and driver software for add-on hardware.

High «kernel_task» CPU usage is due to system/kernel extensions as well as hardware driver software.

Are you running any “Anti-Virus”, “Cleaner”, “Optimizer”, etc. software? Such could easily do such!

By the way, the CPU % is not showing the percentage total CPU usage, relative to total possible, but is only showing the percentage of a single CPU core, equivalent: so, for a four core machine, it can go up to 400%, for multithreaded workloads.

Have you checked your software (and add-on hardware) for compatibility with Big Sur?

One should always perform such a check before upgrading (not a mere update) to a new Operating System (OS).

Here's some food for thought: there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus 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.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is desshoigned solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

With that in mind you might talk with your IT personnel at work and see if you can forgo the Microsoft junk.

As for the VPN as long as you're using it to connect directly to your company's servers it is a good thing. Public VPNs are worthless from a privacy standpoint: Public VPN's are anything but private.  

If you can uninstall the Microsoft Defender temporarily to test its affect on your Macs performance you'll have additional ammo for the talk with the IT people. If they insist on a anti-virus app see if they will let you use ClamX. It appears to be the most benign A/V app in regards to Mac's performance.

Another tact you can take is to boot into Safe Mode according to Start up your Mac in safe mode - Apple Support and see if the problem persists. Reboot normally and check again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Again, more info for the IT personnel. Good luck.

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.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is desshoigned solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

Unless you're using a true VPN tunnel, such as between you and your employer's or bank's servers, they are useless from a privacy standpoint: Public VPN's are anything but private.  

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.

This is very good to hear, Straycat123!

It is also greatly appreciated that you posted these final results for others!