Warcraftlogs Lists separate dps rankings for mythic+ dungeons and the Ny’slotha raid did you notice that. you can also change the raid difficulty, which changes the rankings. How well you enjoy and play a class is generally more important than the rankings. If your good at a lower tier class, you may do better than if your bad at a higher tier class. Specs get buffed and nerfed as well.
Warlocks are the most wanted class in the game for raids. I consider summons mandatory, and they also provide healthstones for everyone. Gateways are nice as well. Locks are generally below average in dungeons. Keep in mind all of the time the rankings will change in shadowlands.
I posted for another lock in this thread for about improving performance with addons. Dot tracking is essential for affliction. Helps for destruction too. Shard tracking for all specs. Eluvi is godly for everyone.