I am not sure if "powerful" matters in this context though. (That is, I expect the chipsets built into TVs to be plenty powerful for their intended purpose.)
We were staying at one just a few days ago that had a cheap Samsung TV. The UI latency was so horribly laggy that simply clicking an arrow on the remote to try to navigate to the next menu would take up to 10 seconds to finally register on screen. It was also variable, meaning some button presses only took 1-2 seconds to respond, but some took 10 seconds, and if you pressed more than once you'd end up with a whole bunch of your delayed button presses registering at once and taking you to a menu option you didn't want.
Sad to say, but state of the art in these Android menu systems is horrible latency, most likely because the UI devs are building in new javascript features that run horribly slow on older ARM processors and they just don't give any F's about the actual user experience or testing...
So it still sounds like a software problem to me.
That would be fine if they were rendering to a 720p screen or had much more simplistic menus like the those found on most A/V receivers, but they’re usually running recent-ish Android or something similar, which has fancy graphics and animations all over the place designed for newer devices which make that hardware choke at the 4K resolution that the majority of TVs now ship with. Exacerbating this are the terrible lowest-bidder smart TV apps which are written terribly.
TV manufacturers will never ship an OS more suitable for the hardware though, because they’re concerned that it will make the TV look less modern than competing TVs. They also won’t ship better hardware because that’d cut $5 per unit off of their margins. As such, it’s best to just write off integrated “smarts” and plug in a streaming box that’s not so anemic.