Travis is much slower after moving to travis-ci.com

Travis seems to have gotten much slower since the move to travis-ci.com
from travis-ci.org in Jan 2019 (#3340).
Now Appveyor routinely finishes before Travis has even started half its jobs.
That never happened before.

I'm afraid that now we're in the same cloud as the private-repo (==paying) customers, we
have lower priority.

I believe that once our jobs start, the speed seems fine: it's getting VM's for the jobs
that is slow.

We should gather some actual numbers for a quantitative comparison.