My guess would be that your webapp is serving images in a blocking fashion, meaning every time a use requests an image it will fetch the image for the user AND block the http serving thread until the image is uploaded. Can you provide more context (eg: tech stack)?
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.
Do you know if the old open street map trick of having multiple tile servers (that are each just aliases of the same server) still works? I think this was how they tried circumventing the 6 connection limit
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
