Hi Jen,
Just another possibility…
I am on my 4th MBP. The previous 2 have suffered from the “too much thermal paste” issue. (too much paste between the processor and the heat sink actually causes the processor to overheat, this causes errors, and it slows everything down.)
It’s a slim chance that this is it.
If you are able, shut the system off until it cools down to room temp and try again. See if it runs at normal speed and then slows down again.
And definitely let us know what it was! 🙂