I think that is perhaps too harsh a statement. I would agree it's not a great experience. It is in no way like the things become like stuck in molasses. But some things become stuttery like the shrinking of windows in Mission Control. When scrolling fast through a PDF, it takes a fraction of a second to load the full resolution, and same for pictures in Photos. Not great, but definitely acceptable.
As I said, and you've further detailed - it's not a pleasant experience.
I can only comment on my experience of a 1.1GHz 2015 rMB. I'm looking at this objectively - you'll notice newer buyers of any product will naturally be defensive of their purchase.
For example, having a consistently smooth UI experience whilst running multiple virtual machines, Xcode and Photoshop on top of regular apps as described above is simply not believable. I work with app developers at work on a daily basis with RRP £3,500+ 15" TB MBPs (we get massive discounts), their UI isn't
always smooth on the built-in screen whilst running less than that.
Perhaps the 2016 rMB offers wildly improved 4K performance, but given the very minor performance increments in all other tasks, I find that very hard to believe. The CPU, by design, is for short bursts of power, ultimately relying on clock throttling to keep temperatures down under sustained load. Running a 4K monitor is not easy on these weak CPUs.
Fact is, the MacBook was never intended for this kind of usage - whether your external monitor is 4K or not.
As I've said many times before, my rMB is my favourite Apple product...but it is not the right product for the OP, or indeed anyone in a similar position.