Delay introduced by transistor
It's called storage delay. Essentially, the base-emitter junction has some capacitance, and when you saturate the transistor, this capacitance is charged. To turn the transistor off, you must discharge this capacitance until you get down to the point where the transistor comes out of saturation, and starts to turn off.

Working against you is the Miller effect, which makes this capacitance look (from the perspective of the thing driving the base of a common-emitter BJT) much bigger than it is.

Some solutions you might consider:


saturate the transistor less
put a small capacitor in parallel with the base resistors
use MOSFETs (so you can eliminate base resistors, decreasing source impedance and thus discharging the capacitance faster)
avoid common-emitter topology
use a baker clamp
use a Schottky transistor (or make one with a discrete Schottky diode)