To give some details: we get 25 concurrent jobs across all the various "official" Python projects on GitHub hosted under the Python, PyPA, and PyCA organizations (which is a substantial bump from what most projects get; see https://travis-ci.com/plans
to get an idea of what we're getting for free). CPython itself uses 3 of those with any single PR or merge into a branch (docs, Py_DEBUG, coverage).
Now normally this works out great for us since CPython is probably one of the more active projects that gets to use this increased budget, and so we typically take a chunk of the 25 concurrent builds happily and get our builds started very promptly. But at the sprints we ran up against cryptography and their crazy build needs: https://travis-ci.org/pyca/cryptography
. IOW having every major Python project using Travis' free service at once hit us hard.