Fast UI makes websites fun to use. Amazon famously increased their sales by 1% by reduce pageloading time by 100ms.
On Wikidata the fact that I have to wait as a user for Wikidata to save a statement or publish a statement reduces the fun I'm having while editing. This is especially annoying when working with the primary sources tool, where the wait time removes the fun of .
Modern Ajax should allow Wikidata to work in a way where the user doesn't have to wait for the server to finish saving a statement to allow the user to continue working.