Sorry for a delayed response, I have been busy with some other tasks.
@hst337 is absolutely right, measuring time in a correct way as well as scrutinizing the same code is the way to test it; I hope to format and put it here soon.
Meanwhile, I implemented the code suggested by @dimitarvp, it is about 25% faster.
Also, I have discovered where Issue #02 happens: after Enum.map(&Task.await/1) finishes there is a noticeable delay (about 20 seconds). It only happens when called from within a barebones GenServer.