Adds some parallelism, makes things slightly faster

These are some "assembly" timings for my project before (serial) and after (parallel):

Serial: 190s, 179s, 144s
Parallel: 137s, 127s, 123s, 113s

Even with parallel, the assembly task doesn't use much CPU. This could mean: 1) The task is disk-bound, and the speedup is largely due to parallelism in disk access, or 2) My case is just weird because I'm on NFS.