Like any tells, physical or otherwise, they aren't always reliable, and they can be expressed in different ways by different players.
Some players don't give off timing tells, i.e. they take variable or random amounts of time every time, or the exact same amount of time every time.
For it to really mean something requires some history with the player, just like with most physical tells. Do they always snap call with a bluff? Do they always take their time with the nuts? Being observant, you can note these details. This is why it is often recommended that you never check the pre-fold/pre-call checkboxes, because it makes it obvious when you've made an "easy" decision well before each hand, versus waiting on the action to get to you to decide. This pegs you either as a beginner or a nit, and observant players can exploit that. I try to always mix up my timing patterns, sometimes tanking on an obvious fold, sometimes snap folding, sometimes tanking before a shove to look uncertain, and sometimes snap-shoving, etc.
But without some history, you can't put a lot of stock in them in general, because of the variety of reasons why people take the time they take. Like you said, they could be pot odds
nits who want a precise calculation on every hand. Or they could be watching TV, on the phone, or just trying to tilt the table. Or in fact trying to project a false tell by snap overbetting to sell the nuts as a bluff. Or, as some anti-multitable whiners like to continuously point out, they could be playing too many tables or involved in a big hand on one table, while running out the clock at yours.