That's a REALLY old CPU. Cinnamon (and Mint in general) is a full and pretty heavy (resource intensive) distro.
Also, much modern video playback looks for (is optimized for?) an instruction set on the CPU (I forget which one) that helps with rending the video in hardware. Your older CPU predates that.
There's not much you can do about this without changing the hardware. You can try a lightweight distro with a lightweight DE. That should help a little, but video playback is likely to be a solo process. If you're doing that, you're probably not going to be doing too much more than that with that system.
You can *try* (assuming you're playing back the video in the browser) disabling hardware acceleration in the browser and, where possible, pick a lower resolution.
Linux can do many things, but making up for time and progress isn't one of them. If you're on a low budget, I'd look for a cheap used computer that's a bit newer.