All queries on the server have slowed down, cpu is normal, plenty of free memory but SQL is using very little
When you're trying to find out why a SQL Server is slow, start by checking these things:


In Task Manager, is the CPU consumer actually sqlserver.exe, or something else? If it's something else, troubleshoot that first.
If it's actually sqlserver.exe, check what queries are running right now. The easiest way is with Adam Machanic's excellent sp_WhoIsActive. Here's how to run it.
Check the server's wait stats. My favorite way to do this is with sp_AskBrent, my stored proc that takes a 5-second sample of wait stats plus checks some diagnostic queries.


For more detail, check out these resources:


Our First Responder Kit - a PDF checklist of things to look at
The free book Troubleshooting SQL Server - I like this one because it's a problem/solution approach. You don't have to read the whole thing to get value out of it - just put it on your virtual shelf and when you've got a CPU problem, turn to the CPU chapter.