Tag slow specs.

In a new project, I would have split these "integration" tests out from
"unit" tests. That still may be worthwhile, but this is the first step
regardless.

You can exclude these specs with `bundle exec rspec --tag ~@slow`, which for me cuts reported spec runtime down to 3.9s (from 6.52s).

As an aside while doing this, I noticed that 2.1 is about 10% slower than 2.0. I tried some naive GC tuning (literally just spamming different settings without actually measuring anything) and didn't make anything better. More investigation required.

@myronmarston