Speed is part of the user experience, but again it's more complex than that.

Nobody would have used snapchat if it didn't take photos but the site loaded in 1ms. Nobody would use it if it had loads more features and took 2 hours.

Speed is a part of it, and must be traded off with all the other parts of the user experience.

I never heard someone say "I use instagram because it's fast"

But how many people don't X because it's slow? I don't browse on my phone's because ads make it too slow.