Timewalking Dungeons are tied to an event where each third week, we can run old dungeons from the group fiender. Our gear and levels get scaled down, but the loot is level 100. However, all the bosses from the Timewalking dungeons also award 1x Jewel of Victory. This has a baseline of 55 Artifact Power per item, and each dungeon awards 3-5 of these, depending on the amount of bosses.
The thing with this is that running these timewalking dungeons is really fast and easy, and a decent premade group can easily chainpull most of the instance, and complete each dungeon in around five minutes! This means that you can almost get one Jewel of Victory per minute, so the amount of Artifact Power per hour is insanely high.
Also note that the base line of 55 Artifact Power is without any Artifact Knowledge, and for example at rank 8 you will get 330 Artifact Power per item used.
When you set up a group for this, focus on bringing classes with good AoE abilities, since most mobs will be group based, and if you can pull 2 or 3 groups at a time and clear with aoe, the dungeon will be very fast.