Thanks for the "All Settings -> General -> Additional Settings > Quick Start+ off" suggestion.
I had been working around this problem, when it occasionally occurred, by taking the power cable out of the back of my TV (an LG-43UM7400PLB) and putting it back in, but I don't want to keep doing that, and risk wear and tear.
So this approach, which worked when I tried it just now, seems much better, and turning the TV on doesn't seem much slower, maybe 10 seconds.