Their damage leveling is pretty weak. I have a couple of videos on my YouTube channel that show the leveling dps and speed as reference. I ended up using a Prot Paladin and 4 Blood Dk as their dps was slightly less than unholy.
at lvl 90 the dungeon grind was no longer worth it, instead switched to WoD quests outside. I didn’t do the actual questing just ran around and grabbed the bonus objectives. With the leveling bonus it was about half a level almost per objective.
Legion I did similar. The invasion quests were good and then mixed it in with some questing just to push through. Dungeons again weren’t as good due to lower dps.

Make sure your using heirlooms.

Even at 120 they still feel pretty weak DPS wise, but I know they can get stronger just haven’t seen the full potential yet.

I did a test last night to try and compare.
Waycrest Manor +8 - 5 druids. Finished in 16 minutes
Waycrest Manor +7 - Pal/4x DK. Finished in 26 minutes. (This was still a +2 with 12 minutes to spare.)

The druids are overall still 15-20 ilvls higher and their corruption is a bit stronger. Once the cloak is maxed and I have Twilight Dev on all DK it should tell a much different story as I’m hoping they are somewhat comparable.