Rethinkdb IO reaches 100% whenever there is a data upload. The load reaches near about 50. Is this a common phenomenon, or do we need to do some optimizations here?
RethinkDB uses a blocker pool to do IO.  On Linux systems, each thread in this blocker pool contributes 1 to the load average while blocking on disk, so RethinkDB sometimes causes the system to report an extremely high load average even under normal load.
Using 100% of your disk throughput is a different story.  If you're running an IO-heavy workload on a slow disk, especially on a rotating drive, then that's pretty reasonable, but it does mean that you might have scaling problems if you want to do more disk-intensive operations.  If you start to have those scaling problems, probably the best solution would be to get a faster disk.
Rethinkdb IO reaches 100%