The Tablo DVR has to tune the station, then transcode the signal so that your streaming device (Roku, FireTV, Computer, Tablet, etc) and stream the show. It also buffers a few seconds of video prior to starting the stream. I have wired connections on most of my devices, but I also have a few wireless. The wireless streaming devices take an extra second or two to start the stream.
I use the highest quality for all recordings & streaming, and I have fast live TV startup enabled. The one thing I have noticed, the stations that have a really strong signal in my area average 3-5 seconds to start, and the weaker stations can take 4-7 seconds to start.
Are these the delay times you are experiencing?