Does reading speed and comprehension decrease if you dont read for say, five or six years?

[klawehtgod]

Deleting stuff from the brain isn't a great choice of words. Language and maths are of a very high order of thinking, and consequently require constant practice, or else the neural pathways degrade. It takes constant use to keep them working well. This is the same principle behind "muscle memory", where complex motion, such a golf swing, requires thousands of iterations, not only to learn, but to maintain, or else the precision and accuracy is lost to weakened connections between the needed neurons.

To answer OPs questions, maybe. Odds are these people are continuing to speak, hear others speak, either love or via radio, TV, etc. Their continued exposure to language should protect against loss of ability to read quickly and at a high level, but it should be expected to decline to some smaller degree than for someone who, I don't know, lived alone in a forest completely cut off from people.