Very slow performance with low cpu and memory usage

I have a refurbished 2017 Mac desktop running the latest version of Catalina. Very little installed, plenty of memory and low cpu usage but still runs incredibly slow, often not even able to keep up with basic typing. I can have everything closed and be looking at the activity monitor, with also seems to have a lag, and see it does not appear to be caused by lack of ram or high cpu usage. What do I do or where do I look for problems? Im not an inexperienced Mac user, but Im not that great with understanding computers so Im not sure how to proceed best. I am a few hundred miles from an apple store so bringing in my desktop is a last resort. Are there any recommendations for third party programs for disk repair that might help?

Any help or ideas is greatly appreciated!