The Radeon R9 M395X while not exactly terrible, is a cut back, low power version of the R9 series designed as a "mobile" version for laptops... not exactly gonna set your pants on fire compared to PC offerings, but should still cause no issues with lightroom. You are however using a rather pedestrian GPU to run a 5k screen, so maybe your expectations are too much. The M395 is pretty much a mid range GPU in terms of performance. Even so.... Lightroom is not exactly like trying to run the latest version of Battlefield on a 5K screen, so no idea why it's slower without it, as it shouldn't be at all, at worst, it should make no difference.. not slow it down, particularly as your processor is not exactly cutting edge either. This must be an issue with Mac OS and Lightroom, not a hardware issue at all.
All I know is that it makes visual rendering, zooming etc, silky smooth here when activated, but then again, A) I'm only running at 2560x1600 and I've got a far more powerful GPU.