Clarification on the compression methods?

On the settings, there's an option for choosing compression method. One of them, LZX, says it's slow but it has high compression rate. Does it being slow just mark the act of compressing it? Or would it also make the accessing of the files slower?

It's likely to also be slower on decompressing, but it depends on your CPU and your disk. If you're bottlenecked by disk, the smaller files produces by LZX might well speed up access, but the converse could also be true. Caching could even make both true on the same system at different times

I do want to add a quick benchmark at some point so you get some idea of how fast your CPU is at each algorithm, but this is never going to be more than a vague guideline. Your best bet is just to try it and see.