In my experiences with all the snakes I've owned (none ever being a retic) is the speed of their tongue flicker depends on their comfort and familiarity of their surroundings (excluding being cold). In the Spring I come across a lot of wild garter snakes trying to get some warm sunrays. It surprises me how cold it is and they are out basking. The tongue flick rate on these guys can be pretty slow and sometimes they won't even retract them all the way.
All of my snakes when they are new to me give their tongue a workout. As they calm down over time and their security feels unthreatened, the tongue flickering either slows or more commonly less frequent flickering.
There is tons of research out there on a snakes senses. While some research will disagree on how well a snake can see ( my experience is if there were glasses for snakes they would all be wearing them), the research all agrees a snakes hearing is poor and the flickering of their tongue is the most important sensory item a snake possesses. So important, it has it's own sheath inside of it's mouth.