What are you talking about? The J-20 left ground faster than the JF-17 and only slightly slower than the Taihang equipped J-10B.
I mean it got airborne quite quickly (yay) but it didn't make much of a climb afterwards (and JF-17 should have a shorter take off if it wasn't weighed down by fuel tanks, but I digress).
J-20 once it flew a distance looked like one of those triangular ufos lol, partly due to its colour and partly cause few other planes have a similar configuration. Pretty awesome.
EDIT: actually I take that back, the J-20 did make a pretty decent climb >_> Angle of the video confused me first time around.