I'm developing a tile server to show some things over Google Maps. Currently, I'm returning PNG images. I have noticed that my request time grows: the first requests are finished earlier and the last requests take a little longer. I'm currently doing 25 requests at the same time in a normal browser. See the network info:

As you can see, the request time per se is the green bar, but there are a lot white and grey zones and that's degrading doing my performence. Here you can see the description on the highlighted request:

The queue or stalled time I suppose is due to the limit of concurrent requests that can be made in a browser (I'm using Chrome, I think its limit is 6 requests). But then what it's causing the other time? To solve the problem of the concurrent request limit I have read that I can use multiple subdomains pointing to the same host.

I'm using Image objects to load the PNGs, setting its src attribute. Do you know what may be causing those delays? Any solution?