Turning off Fast start will cause the set to completely power down when you turn it off, instead of just going into standby mode. It will then have to do a complete cold startup each time you turn it on. Most users keep Fast start enabled so the TV is ready to go when you turn it on.
One other thing to try:
The equivalent setting to this on Roku devices is Settings > System > Power > Auto power savings > After 20 minutes of no interaction. (I'm not where I can check right now, but I think the wording on Roku TVs is slightly different, 15 minutes). On Roku DEVICES this can shut down the system to the point that it drops the network connection, but I don't know if it has the same effect on Roku TVs. If you have this enabled, try turning it off to see if it helps.
