15 seconds might be for the self-hosted client-side-rendered version. I agree it's very slow, and it can and will be improved, it's just not our priority at the moment. 

Our architecture is quite different from Nostrudel, our pages need the full data set loaded to generate proper HTML using a theme, vs Nostrudel that has no themes and can update parts of the page while receiving new events and thus can start showing you stuff much faster. We also have much higher expectations for coverage - if Nostrudel misses some events you won't notice and won't be too mad, if we don't show one of your posts on your site - you will 100% notice and report a bug. So we can't just copy what Nostrudel and others do, we have to figure out our own way of loading the necessary data as fast as possible. 

We'll get there, but it will take a while.