I have the most recent MacBook Pro before the change, 2.8 Ghz processor/16GB ram/1TB SSD. I just received my new MacBook Pro, 2.9GHz/16MB/2TB SSD.
The two machines are virtually identical, since I used migration assistant. Lr library and images are on a second partition on both machines, but the library and image files were identical (cloned over with Carbon Copy Cloner). I set them side by side and tried to put several tasks through the paces with Lr at the same time. Obviously I can't do simultaneous brush work, etc. but rendering a Lr Stiched pano from 5 80mp files in the develop module up to 100% seemed nearly identical, the new one perhaps slightly faster. Moving around in the Library and zooming to 1:1 on various files is a push. Selecting the same brush on the stitched pano on both of them, something with probably about 50 or more brush strokes cleaning up edges etc. then showing the mask overlay the new one was about a second quicker the first time, but then toggling off and on it was more like .2 seconds faster.
I then made a virtual copy of that file, deleted all the brushwork and worked on creating it again, doing the same thing on each machine. The new machine seemed perhaps slightly more responsive, it definitely wasn't more sluggish than the old one. After about 5 minutes doing the same thing on each file (the history showed about 50 brush strokes and a few other things) I didn't feel there was a significant difference. I had the GPU enabled on both machines.
So it's about what I expected - not much difference is speed and certainly not sluggish or problematic. Maybe I haven't tried to do functions that others have found cause the problem, not sure. my main interest was the new display, something I haven't been able to really get into yet.
The dongle thing to me isn't any big deal, I think most will opt for a little 1x3 inch "dock". I've got one coming that turns one TB3 port into mini displayport, USB-c pass through charging, SD card and a couple of USB ports. The dongle for USB 3 is only $8 right now. In a year or two, I think USB-C will be embraced pretty quickly by most, and I wouldn't be surprised to see it even begin showing up to be used instead of HDMI on most displays and TV's.
My biggest complaint is Apple not having a solution for mini displayport when they released the machine, even if it was another little dongle. Many displays now such as my NEC run displayport, but I have to use HDMI because there isn't a good solution for mini displayport. But all of it short lived, the new connector promises to lead us to a world of one connector to handle all devices. whether it pans out I'm not sure, because it seems while all the connectors are the same, the cables may not all be interchangeable. Maybe better than were we are now, but it would be amazing if the cables were all the same, the connectors were all the same, and you just hooked things up.
One other big complaint, the new MBP when used as a target disk through the TB3 - TB2 connecter has terrible speed, only gets about 130 MB/s. The previous one was around 500-700 MP/s. I use my laptop as my work drive all the time, when at home I just set it up as an external drive. The new one is sluggish when I do that so I'll have to migrate the library back and forth. Hopefully it's something they can address with a firmware upgrade to the dongle. They've already updated the HDMI/USB dongle once.