What's blocking is number of http connections between browser and server. Most browsers only allow 6, each tile takes about 100ms so getting 10 in under second doesn't always happen.
Your webserver still has to spin off a thread for each request if you want to do substantial CPU work for each request, but rest assured you'll get all the requests at once from the browser. Not 6 at a time like in the dark ages
The RFC recommends at least 100 streams. See SETTINGS_MAX_CONCURRENT_STREAMS https://tools.ietf.org/html/rfc7540#section-6.5.2
https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Images/...