The instant on feature is generally just a standby mode, which can consume a fairly substantial amount of power.
I've also found that disabling the 'instant on' feature of my TV makes it perform much better. When I used the standby mode, apps would occasionally crash or stop responding after a few hours. The only way to get them to work again was to unplug the TV.
I never have to unplug the TV now that I've disabled instant on.
That being said, I own a Samsung TV, which has the worst software of any TV I've used, except for Philips, which was worse only because the TV I owned didn't support SSH keys longer than 48-bits or something really stupid like that.
Ideally I'd be able to purchase a non-smart TV that turned on instantly, but I don't see that happening any time soon.