FYI, Heroku also offers 750 free hours per-app.
If you want to run a celery worker (including beat), you're looking at a total cost of 30$ per month. That's enough for most moderate traffic websites. :)
Our process model is really only measuring application entry-points. You could have your free "process" spin off 10 other subprocesses beneath it. The only constraint is RAM.
I've been scaling mine around, and it's never taken more than a second or two. Although, maybe that would change if you are scaling up to hundreds of workers or something like that.
It's common in free versions of such services, to only load things on demand, which is a little slow --in Google App Engine you can pay to have instances pre-loaded.