TL;DR

Slow Clients Are Problematic

Most benchmarks we've seen don't tell you this, and unicorn doesn't care about slow 
clients... but you should.

-- The Philosophy Behind unicorn

Why

In my experience, a lot of companies use unicorn as their rails / rack app server, because it's "fast", but they have never understand the problem of "slow client", and IMO most of them should have, as Ruby and Rails are not well known for performance.