1: Wait until Toady optimizes stuff. It won't involve multithreading, though, because it would soak up 2 years without new features.
2: Get a graphics pack.
3: Not as important as new features, in both my and Toady's opinion.
1 : Well, development can be speed up by actually getting help from other people who knows how to add multithreading to workloads as what you have in Dwarf Fortress, several areas could be done in separate cores, now I'm not an expert here but you could have one core that coordinates world events and temperature effects in the area, one core for handling calculations of certain effects like when units move around, attack, catapults throw things around, water flow , magma flow and so on. It needs to be coordinated yes, but I think ( again not from an expert in this tema) that it could help out in a few situations.
2 : Well, for someone experienced it's not "difficult" to add an graphics pack, but from those that pick up the game, tries it out for the first time and doesnt understand anything and just goes away its a big and important point in case Toady wants to maintain potential players and donators.
3 : Sure, new features are nice, but when the game goes to an halt on a system that picks up some of the bugs or the bugs are game breaking it means the people playing the game and possible donators for future development just goes away. Just check out new games, if they have many bugs people are more unlikely to want to pick them up and buy them...
I used to play this game quite a lot, started around 2009 and donated a few times ( would have done more donations if things actually where fixed on some areas and a few other reasons ) , but lately I've found the game to be less attractive since the same bugs are still there that broke part of my experience.