I mentioned this earlier this year in January here.
At that time it was just 1.5 seconds, now the fastest it can go is 4.x seconds with all products finally added to virtuemart. The client is very disappointed by the slow loading times.
Watching the network-tab in Chrome it is the actual document which is slow (the megafilter component page), not the loading of additional json files. The json file takes just 39 ms.
If the json file is loaded after the document is loaded, why is the render time for the page increased with more products in virtuemart? Is the component doing something else in addition to just reading the json file?
I've tested with and without cache enabled in Joomla but it doesn't help at all. Might just shave a few milliseconds from those 4 seconds. So what is causing the component to render so slowly? After it has been loaded all the javascript takes over and loads the json etc. Which is a lot faster and hardly noticeably.