Let's consider his conclusions:

    ruby is slow:                         sound
    ruby's slowness is his main problem:  not-sound

He's not going to win programming contests with brute-force solutions. Even his brute-force method makes unnecessary computations - why keep running after n^2 is out of range?

> memoizing, storing values/lookups that would be used later, limiting search spaces, etc

He did none of that. Kudos to OP for benchmarking & profiling though -- the speed of Go is impressive.