Dude, multithreading... you gotta do it. Even Krita is starting to work on it. Multithreading is pretty much vital to performance in modern day software now that everyone and their dog has a 4-8 core processor with 4-16 threads. Without multithreading you're tied to 1/4 of the max perforamnce of the CPU, 1/16 in the case of the most recent high end processors.
Besides, isn't it an excellent opportunity for you to grow as a programmer?
But my thoughts on saving, is that you should enable background saving so that it doesn't freeze the entire program whenever you save or autosave, this could indeed by solved by making saves happen on a different thread right?