Laptop started beach-balling suddenly

Hello!

My mid-2012 MacBook Pro (running Mojave) all of a sudden started lagging and getting the dreaded beach-ball after every click. This just freezes up my laptop for about 15-30 seconds before the click is registered.

I was working on a web page, closed my laptop, and when I came back a half hour later everything was slow as soon as my laptop woke up.

I've replaced my hard drive with a Samsung EVO SSD and it has been running beautifully for 6 months, but this started out of nowhere. I've run First-Aid in Disk Utility and it shows everything as working, and I'm not sure where to go next.

Any advice on what the problem may be or how to investigate further would be greatly appreciated!