On my MacBookPro, the UI is normally responsive, but if an image export with DP3 is running in the background (more often than you might think, since it takes around a minute to process the larger RAW files), then the entire UI becomes unsuitably responsive.
As others have pointed out, disabling the higher resolution previews (especially the DP3 noise reduction preview) mitigates this and also makes moving between images faster.
When there is no export in progress and no DP3 full-size preview enabled, I do not see any lag even with GFX100s files.