Following instructions found here, I ran the HPL benchmark on the four compute nodes of the cluster.  On the first try, I'm getting 390MFLOPS.

That's 300 million times slower than the fastest ranked supercomputer as of June 2017.

However, it's at worst only 40% less efficient* than the least power efficient computer on the same list, so that's nice I guess.

*Didn't measure average power during the run, but it does have a 5V 2A power supply and isn't hitting the limits.  So I'm assuming 10W running full out.