When you say a "slow server", is it every query that is slow? Is it a specific stored procedure or view that is slow or everything you do on the server?
What does the CPU and memory in use look like (perfmon will help you monitor that)?
Are you sure it is the database that is slow and not the app or client machines accessing the app?
If you have a test version of the database, I would spin up profiler or an extended events session on the database, connect from the test version of the app to the test database with minimal connections (ie just you) and see what is actually getting run against the database and how long the database queries are actually taking.
I had our RnD guys come to me telling me the database was slow before and when I dug into it more, it turned out they needed to redo some indexing stuff with elasticsearch on the server and nothing needed to be done on the database side.
Failing that, I would reach out to the app vendor to see if they have run into something similar to what you are seeing. It could be something simple like compatability level of the database or a new version of the app, or it could be something a bit more complicated. Software vendor would be my first stop though; they know their applications and will be the most likely to point you in the right direction. If they prove to be less than useful, I would try to narrow down what exatly is slow to the best of my ability (SQL, app, network, disk, memory, CPU, etc).
If it is a VM, you could always ask your server team to toss more memory and CPU at it briefly as a proof of concept. Looks like you are configured to use roughly 122 GB of memory. Do you have enough free memory for the OS and other things running on the server (antivirus, other SQL instances, etc)?