What are some of the functionalities I can turn off to speed up an old Apple Watch?
The older watches are constrained with RAM and CPU.

To help with RAM - be brutal and remove every app you can. Only install things you need. I still have my original Apple Watch and it is a bit painful when I swap it in instead of my Series 4 - but it’s still sentimental in a way my original iPhone is.

You can’t really change the CPU time it takes for apps and complications to run, but by removing things and not running them, the RAM can get used for the things you need.