Next time you launch the game, make sure that on the launcher settings you have Hardware renderer enabled. Launcher settings can be accesses clicking the options button in the right bottom corner before clicking the 'Play' button. You can also increase the amount of max RAM there allocated for the client if you want to, but that shouldn't be necessary.
Next when you login or when you're on the login screen, go to the in-game settings and make sure you have these selected:
Those settings should give you the best performance no matter the system. If you game is still running slow, it's most likely because your PC is simply way too old. The game should look somewhat like this:
If your problem can't be fixed though; it doesn't really make sense to continue developing for computers that are obsolete.
If the majority of the audience has shit PCs to begin with, it's not really worth developing for an audience with great PCs if such audience doesn't even exist. You would be surprised how shit PCs people playing OSRS have.