The fast live tv startup setting is more for buffering issues not so much for speed. An excerpt from a reply posted by TabloSupport:
That said, there really isn’t too much to this feature. With fast live TV startup enabled, the Tablo sends fewer segments to its devices and starts the stream earlier. For some networks and devices, this buffer is too small, and can lead to buffering - in some cases, it never recovers.
The speed is often determined by network topology and congestion. The device you “watching” with, it’s doing the processing once it gets the stream from the tablo. How many different shows are being steamed and to a lesser degree, how many are being recorded. The above referenced post goes on discussing network speed and Roku bitrate.
Even a TV tuner seems to take 2-3 seconds to change channels, then transcoding, buffering a couple seconds… steam over a home network, next device to process video. A few seconds, which is too long just going through channels