Gecko navigate performance when running multiple instances

I develop and applications that is basically a windowless GeckoWebBrowser. It has a method that performs Gecko Navigate and is async. For tests I create several processes with that application and make them navigate to same site.

So if I run single application - it takes roughly 3 seconds to load the site. If I run 20 of them - the loading time increases up 30 seconds (my timeout) for some of the processes.

What could it be? I mean could it be that Gecko/Xul somehow shares something between those applications that makes stuff so slow?