Nothing to do with the mouse, as proven by changing it.
Problem comes from something hogging system resources in a big way, something in memory.
This could be your AV program scanning in the background or checking a site you are on, something in one of your programs scanning stuff on your hard drive, or a host of similar operations.
It could be a program you installed has a bunch of stuff that runs in the background. That one is wide open for what it may be doing. Some programs have a lot of junk built in to them.
Did you install a new program around the time this began? That would be the first place I would look.
Anytime there is background activity sucking up memory or resources, the mouse can act like this.
Bring up Task Manager by hitting Ctrl/Alt/Delete. Click the Performance tab and see what the CPU usage is at and watch it for a minute to see if it spikes way up. If it does, that would be the time the mouse moves slowly as you describe.
Also click Processes tab and look at System Idle Process. Look at number indicated. A high number indicates how much of CPU is idle. If this number drops way down a lot, something is hogging resources. Or, while in that screen, look at bottom of that screen and read the CPU usage in percentage. It should be relatively low. If it spikes way up, same thing, resource hog going on. You may have to click the "Show Processes from all users" box.