PSO2 on Linux casefolding guide
This guide is primarily written for the Global Steam version of the game, however it should apply more or less the same with other versions, including the Japanese version in case it starts allowing Linux gameplay as well. (JP currently does not have an up-to-date version of GameGuard anticheat).
I do not hold any liability for what happens if you mess up, be warned.
I would recommend reading through the entire guide even if certain parts may not apply to you.
PSO2 has a unique structure where each assets are contained in their own tiny files, mostly ranging around 3-4 KiB in size, and would even try to load assets that may potentially exist even if it's guaranteed to not exist, in turn this adds quite the considerable overhead for Wine to try to load these files off disk.