Joined: Aug 2010
Posts: 12,760
Likes: 2,117
From: Madison, WI
Bikes: 1961 Ideor, 1966 Perfekt 3 Speed AB Hub, 1994 Bridgestone MB-6, 2006 Airnimal Joey, 2009 Thorn Sherpa, 2013 Thorn Nomad MkII, 2015 VO Pass Hunter, 2017 Lynskey Backroad, 2017 Raleigh Gran Prix, 1980s Bianchi Mixte on a trainer. Others are now gone.
Slower riding I would expect would make your saddle less comfortable. But otherwise I would not expect anything else to be hampered by slower riding.
I find that when I ride harder I am more careful to time my caloric intake with my body needs and more careful about hydration. Perhaps if you are slower and relaxing more you are less careful about being properly fueled for a long day?