One of my front-end developer co-workers asked for my help because of the browser not sending HTTPS requests asynchronously. At least that is what he thought at first. By opening up the Firefox networking tab, I realized that the requests get sent asynchronously, but for some reason, only six HTTPS requests are allowed to be sent parallel.
I assume this is a technical limit of the HTTPS protocol, though I do not know the cause. I searched for the cause on the web, but I have not been able to find it so far.
In the following picture, the red parts of the lines mark the duration spent being blocked by another request: