Why image TTFB gets higher always on full page load, but not on single image request

I have a very simple e-commerce site that shows like 50 images on its home page, in a list of products.

This site is on a stage server, not production, so it's not under load. Resources are pretty high, 32 GB of RAM and 16 core of CPU and stuff. The point is, we test this on a very dedicated server, with only one client browser. So, slowness can't be because of the resources of the server.

But the problem is that, when we refresh our site's home page, we see that images' TTFB gets close to a second, while each image is loaded separately will only be server in less than 100 ms of TTFB.

We know that it's related the batch operation somewhere. Either in Browser as User Agent, or on the server. But we know for sure that it's not related to server resources. There must be some sort of configuration somewhere that prevents this.

How to debug this issue? We're using .NET platform, and Windows server, and IIS.