This is a follow up of bug 18654 that introduced the throttling for download progress event. The specification states that they should be dispatched in the same fashion: "about every 50ms or for every byte transmitted, whichever is least frequent." (see http://www.w3.org/TR/XMLHttpRequest2/ section 3.6.6)
*** Bug 37771 has been marked as a duplicate of this bug. ***
Code inspection shows this should have throttling now.