From National Geographic (all true):
In Nature: While snakes are often perceived as lazy, Campbell said people are mistaken. "What we perceive as shy, lazy, or inactive is really efficiency," he said."On average, they are bigger than other lizards and can build a lot of body mass. They do that by being efficient in feeding and traveling." In other words, snakes don't move very much because they don't have to.
When it comes to food, snakes catch prey that are significantly larger than them so they can eat less frequently. This reduces the time they spend hunting and thus makes them less vulnerable to falling victim to a predator themselves.
For Campbell, the most impressive hunter is the eastern diamondback rattlesnake (Crotalus Adamanteus), which is able to hunt and kill its prey very quickly using venom, so it doesn't have to travel far. "Because they don't have to do that, they can become relatively large and heavy, being able to build up body mass and not having to spend that energy hunting."