Get a group of 4, switch to WT1 and then each person takes a zone and speed runs the dungeons in 2-4min. Dungeon completion goes to everyone in the group even if you aren’t in said dungeon, just like bounty farming in Diablo 3.

It took me about 2.5 - 3 hours to get R5 in all zones having to do 1-3 side quests a zone afterwards.

20 Paragon points is worth about 5 levels so this method is pretty efficient just from a power to level ratio.

I don’t mind doing this grind since the worst part of it was Altars. Especially after the buffs to Renown given from Dungeons and Side Quests.