Actually, I was looking at how audio loops work, and it seems like the low context switching overhead of fibers could be really great for stacks of effects and filters. Because the overhead is very small and predictable, and the ergonomics of fibers is easier to deal with, it could make for a really nice interface.
Even if the background process is I/O intensive (which is supposed to be most of the time waiting, therefore freeing the CPU for the foreground process), it doesn't mean it won't end up still blocking (I've experienced this with filesystem operations).