Does the starter seem to turn the engine over the same as it once did?

I would see how the starter spins the engine with the plugs removed. How does it turn by hand, using a wrench?

At this time, you could do a compression check.

I would suspect damage to the bearings or other internal engine parts, causing extra friction.

As for the thermostat, I would remove it and test it in on the stove. Put it in a pot of water and see if it opens when it should - i.e. before the water boils.

This is quicker and cheaper than installing a new one.