What do you mean by loaded on demand? You can instantly scale up and down the amount of workers you have, if that's what you mean.
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.
He probably asks if they are pre-loaded (so when someone first visits your webpage they don't have to wait for the django config to be parsed and it get loaded onto memory).
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.
Oh. Yes, they are identical to paid dynos. Essentially, Heroku just deducts 750 paid hours from your bill each month (which is just enough to run one web dyno 24x7).
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.