Rattlesnakes have a highly-efficient digestive system which takes a lot of metabolic energy, as does searching or prey.
After a rattlesnake swallows its prey, it will normally hide out while the meal is digested.
Rattlesnakes become sluggish while digesting, a process that can take several days depending on the size of the meal.
They average about two weeks between meals, when food is available.
Young rattlers eat much more frequently.
Perhaps you have mistaken them for Boa Constrictors, which do have a relatively slower digestive system., since they are ambush predators who lay around and wait for food to happen by, rather than actively hunting, like rattlers do.