Here is what TabloTv says about that setting (back on May 5) Enable Fast Live TV - #12 by ncwaterman
Fast startup changes the length of the first few segments fed into the video stream. In ‘Fast Startup’ there are two ‘short’ segments followed by two ‘regular’ length segments. When it’s disabled you get four regular length segments.
Some network setups can handle the shorter segments and some cannot which is why we added the checkbox. If it works for you, great, if not it’s likely caused by slower network speeds or other networking issues and not the Tablo software itself.