While a user is being edited, the username's existence is checked every time something is typed. It can very quickly queue 5 or 10 HTTP requests, and since the responses will arrive with a delay, the effect on the interface are sometimes erratic (field marked as invalid for a second, etc).

This could be improved by sending this request with a sliding delay (like 1 second, maybe more). If the text is changed during this second, the delay is reset to 1 second.