I've looked at, and read up on, a couple of these other sites, (500px, ipernity, etc). On the one hand, apart from the factor of competitive software updating, any of them is probably as good as any other for file sharing. But when it comes to groups, other than special interest forums, Flickr is still where the majority of folks seem to be, a factor that, as with Facebook, can be more important than the software design itself.
Even if it is now impossible to find them or view their shots comfortably !!
I haven't seen an issue with fast or slow loading with Flickr, I just find it irritating that they load my contacts' images so large when I only need a thumbnail to get an idea of what I want to look into. So I now look at 10% of what I previously did. The Flickr designers have effed up big time, but I assume that it will change again in the future, and hopefuly with more options for personalisation. Flickr will remain my dumping ground.