It's more than just the two piece uptime being reduced. It's the entirety of DRW being reduced, meaning your primary tanking ability and mitigation tool uptime is reduced.

It's anywhere from 34~% less with perfect play to... Well really the sky is the limit. If you already weren't successfully hitting high end stacks you're probably going to be well beyond that mark. The nerf is drastically heavier the worse you play, and even with perfect play it's spec-killing for most players in 24,25,26+, especially on Fort weeks.

If you want to test out how it'll feel, you can effectively take off your 4 piece and run keys with just the legendaries and 2 piece.