I've had great success blackholing and haven't seen this.
I have an LG I bought around three years ago. The first year-ish it was connected to the internet normally. That's when I first heard about the LG spying issue, and also had noticed that the TV seemed to slow down a bit with every new update it got (what a surprise).
I then isolated it to its own VLAN with no internet access whatsoever. Only other device on the same VLAN is a virtual interface on my Home Assistant server that allows me to keep using the LG webos integration and send Wake on LAN packets to the TV.
Surprisingly most of the slowness seemed to also disappear after cutting the TV from the internet. It's probably not quite as fast as it was when it was brand new, but the most noticeable difference is when turning the TV on. It's almost instant again now, compared to something like 5+ seconds it took when it had internet.