There's no need to "stutter". Weston can use OpenGL to stretch current window contents if the application cannot provide a new buffer on time. This is a better solution than Haiku IMHO, because you do not show half painted windows and can be hardware accelerated.
Rendering in a thread doesn't make drawing code magically faster. It only solves the problem of the application missing the retrace deadline because it's busy doing actual work, at the price of doing less such work (the thread needs cycles to draw the window, too).