I dont have any problems keeping up as a demolock. I mainly play m+ and I out-dps any affliction warlock I come across. Yesterday I did a 10 underrot (tyrannical, explosive, bolstering, infested affixes) and 2 chest it. Im ilvl 375 equipped.
My overall dps on the run was 14k. Demo is insane for constant cleave aoe with imp exlposion. When chain-pulling in m+ and quickly go into boss fights after trash, Im having 2-3 instant casts ready before the fight even starts. Save them up for the time when you need to move. To me the spec is really mobile. Yes, you have some ramp-up time at the start of the dungeon or when combat has left for awhile (after a wipe perhaps).
Not to mention the insane stable ST dps. On the first boss I did 18.2k dps at the end of the fight. All other bosses I was around 12-15k dps. Sure, the first boss in Underot has a little cleave for a few seconds, but 3rd and last boss dont have any cleave and require alot of movement, and still Im doing 12-15k dps. Ill see if I still have my details from the run to proove what im saying.
Point is: demolock is a pretty solid spot. Get to know the procs and keep them for when you need to move so you basicly have no downtime. Constantly pumping out instant demonbolts critting for 30k is really good.
Edit: Posted the details from that run. Showing you the Overall dps, the 1st boss and 3rd boss
This is the link to my wlock so you can check out the gear and recent runs:
https://raider.io/characters/eu/kazzak/Conzise