Much like a computer, it is only as fast as it's slowest part.
In the case of XenForo that would appear to be deferred.php Even on XenForo.com running the file by its self is still 1/2 a second (between 200 - 500 ms)
Which of course calls upon
This seems to vary depending on how many people are online & I would assume whatever process is being run... ie... post, edits, search, ect....
But knowing where that bottleneck is means knowing what needs to be improved. And with it being only in or about 1/2 a second, it's not like it's mission critical or something to really make a fuss over.
Of course if you disagree with my opinion.... Where would you believe xenforo could be improved upon (where is it's slowest part)?