First, get a complete and current backup, if you do not already have one.
This is important: get a backup. Now.
If you do not already have a complete and current backup of this Mac, get one Right Now.
This because you might have failing hardware.
Specifically, a failing hard disk drive is a possibility.
A common thing many folks try—when presented with failing hardware—is running diagnostics, maybe reloading apps, and file system repairs, and tests, and, well, generating lots of I/O. To what might be failing hardware. I/O which a failing hard disk can limit, as it continues to degrade.
Choose your I/O usage wisely.
Choose a backup.
Once you have a complete and current backup, download and install and run EtreCheck. Within EtreCheck, share the report to the clipboard. Then open a new reply here, press the button that looks like a printed page to get a text input box big enough to paste the hardware and software configuration report here. With that report, we can tell more about the condition of your system, including whether the storage may be failing.