Right off the bat: as a long time Shield user I actually do like the new interface but I do think with all the extra tabs you get with the CCwGTV that it's causing it to slow down. It feels like every time i turn on the CCwGTV there is a good 30 seconds before it's useable. I'm guessing during this time it's refreshing all my recommendations. Others seem to have the same issue.
After getting the update to the new interface on my Shield i notice no such behavior but chalk that up to the Shield being more powerful.
I picked up two Onn. 4K boxes for my in-laws place and have been playing around with them. Honestly from a interface standpoint it's just as snappy as my Shield.
This does give me hope that Google can fix the CCwGTV performance with a software update.
The thing that really annoys me about my CCwGTV is related to one you mention. If i do not fully back out of YouTube before the device sleeps or I turn it off, when I activate it again the YouTube app just sits there frozen for about 30 seconds because it can't close it normally.
Plus it's become incredibly stuttery while browsing no matter how much free space I reclaim. I'm glad I bought a Shield and put the CC in my bedroom.