Forgot to mention general drawing quality and background drawing quality... but this effects visuals in a big way, so only resort to it if you are desperate. (I can get the game to run on my laptop which is pretty outdated... but it looks like a kid ate some crayons and threw up on my screen.)

also texture quality and texture filtering are pretty standard ways to improve performance though again it will effect your visuals too.

All of these options are found in the config program, seperate from ffxiv itself, if you are having trouble finding them. (I am sure you already know that, but I don't want to assume and then leave you lost while trying to look for these things.)