Owner of a 4K Sharp "smart" TV here. Menu is indeed incredibly sluggish. My old Zenith from the 90s has better response time, _and_ an integrated VHS player. Can't beat it.
The Sharp was the first smart TV I ever purchased. Heavily discounted at a holiday sale some years ago.
When I brought it home, I connected it to the Internet to see what all the fuss was about. They're nothing special -- you can launch your typical streaming, news, and weather apps, all from the same terrifyingly-slow UI.
A few weeks later, the TV forced an update on me. A pop-up appeared _on top of_ another (external) video source, interrupting what I was doing. There was no option to decline the update, not even "remind me later".
I decided to turn the TV off rather than be strong-armed into the update. But when I turned it back on, the update popped right up again. I tried blocking the TV's Internet access using my router, but this had no effect either. At this point I felt mostly defeated, and assumed I wouldn't be able to use the TV until I "consented" to the update.
Needless to say, the TV is not allowed to connect to the Internet anymore.
Another complaint about modern smart TVs: the remotes are horrible. I don't know if they're IR, Bluetooth, 2.4GHz radio or what, but not only must you have line-of-sight to the receiver, you must point _directly_ at it, or some inputs will be dropped.
The remotes also often have dedicated buttons for proprietary streaming services which I've carefully removed with a razor knife. There is absolutely no reason for the remote sitting on my coffee table to function as free advertisement for Netflix/Amazon/Hulu.
I understand these buttons on the TV UI, but branded buttons on the remote control? Come now. That's going to look quite silly in 15 years when some of these services are defunct, or when the TV's OS is out of support and the apps won't run anymore (like what happened to Netflix on the Wii, or YouTube on the PS3).
This is assuming the panel even lasts that long... given all the R&D that manufacturers must be pouring into unnecessary software, I can't imagine the longevity of these devices has done anything but shorten.
I continue to search for large, high-quality "dumb" panels that I can purchase, but it seems like these are mostly nonexistent in the consumer/retail space. Any dumb TVs I find are usually very small. Everything over 35" or so seems to always be a smart TV.