Often it does not do everything in one job. It’s split it up into a smaller jobs. For example, it insert 100 products each time, instead of 10000 in one go.

The plugin does not split up jobs at the moment, the worker will just run for all 10.000 jobs, but if you're using the Cron setup, it will do it behind the scenes. (It will still take a while, but at least people are not waiting for it, and the work has to be done anyway, or else you shouldn't queue it in the first place.)

Instead of doing what I suggested with starterhook or the image hack, it could probably be solved with an ajax call.

Sure, that's the same thing. The important part is that you cut the connection to the client as soon as possible, and ignore that connection loss. You just want to trigger a work script, you are not loading something for the user. What if your job takes one minute, the user closes the page after 30 seconds, and your script terminates mid-work? If you're using an image or an AJAX call: return something, disconnect, then work.

Edit: note that the cron job is always preferred above the AJAX or img, because if you don't have visitors, you're job will not be done with the AJAX or img route.