The best feature is that it's not slow at all! It has been working flawlessly since I got my first one in 2015 (it was then called DGN2200).
No problems with speed or reliability whatsoever - so far anyway :-) There isn't anything to dislike about this product as yet but if there were any improvements from their side they would be very welcome indeed!! We have multiple sites across 3 different countries which need protection via our firewall; we use them primarily during work hours when internet usage can get quite high without us having set up specific rules specifically designed around such times of day e.g., after 5pm / before 8am etc..