Howard Miller

It might not matter... keep going. The caches get cleared as part of the upgrade and that usually sorts it out.

.dll are exclusively a Windows thing. Nothing to do with MacOS or Linux.

Why Xampp in particular? I mostly do development using an M1 mac. I use a Linux VM to run Moodle in - I'm not necessarily recommending that; just pointing out that you have options.

Have have you done, if anything, to attempt to localise this problem?

This can be where good server monitoring software can pay off. You need to understand what the server is doing while it is "slow". Are these "on premises" servers or "cloud" hosting. What, by any metric you like, is the "size" of your site (how many active users or similar)?

Have you run Moodle's Performance Report. It's basic but still worth running. 

I would be checking what the database is doing and what Redis is doing. For Redis also, if you can, check the network (Redis generates a lot of traffic and can swamp the network). Check very carefully that Caching is correctly configured. 

Check sessions handling - is this using Redis as well? Do NOT use the database for sessions. 

I don't know anything about Windows / MSSQL so cannot be more specific. 

They could just be old accounts that no longer have emails in your system. They will cause errors that look like that.