The feature keeps more ready in standby
Roku explains that Fast TV Start supports quicker return and specified audio or voice functions while the TV is in standby. It also says the setting uses more power than normal standby. Those are linked parts of the decision.
Choose it because a supported task matters, such as starting audio with the screen off. Do not describe every faster standby state as an automatic energy-saving improvement.
Compare the ordinary household action
On a supported Roku TV, inspect System, Power, Fast TV Start and the model’s instructions. Test the actual wake or audio-start action with the intended remote. A feature in the TV does not give an incompatible remote hands-free capability.
Keep the same speakers and controller during the comparison. If the screen wakes quickly but an app remains slow, record those stages separately.
Keep the conclusion practical
The useful outcome is whether the feature makes the intended action reliable enough for the household. No exact energy or startup-time claim follows without measurement on that model and arrangement.
If the extra standby behavior is not needed, use the supported preference that suits normal use. Recheck after changing speakers or control hardware because the intended task may have changed.
Questions answered
Does Fast TV Start use less normal standby power?
Roku says it uses more than normal standby.
Does a fast TV wake prove the app has already loaded?
No. Display wake and app readiness are separate stages.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.