I would like to revise my statement. After some much more serious work in LR, I am seeing significant slowdown in a number of areas in Develop while at 5k resolution. Interestingly, some similar-sized images seem to do better than others.
There are some clear problem spots/culprits.
For example, toggling soft proofing on and off ("S") triggers occasional beach balls and does not reliably use the same amount of time on each cycle. This is definitely suggests that something is amiss, because it should take rough the same time once in steady state. I don't see any disk activity (paging out memory, re-loading the raw file) that would serve to inject random delays. It's weird.
Spot removal, always one of the more expensive/slow tools, performs especially poorly. Executing an undo on a spot removal sometimes takes 5 seconds or more.
I think Adobe definitely has some work to do. I'm a little surprised this made it through QA