My friends start counting 2.6 seconds as soon as they turn over a monster and combat begins. That's not even enough time to read the card! Is this legal?

The rule says you must get "a reasonable time" to respond when another player is winning a fight. What your friends are doing is not reasonable; you should be able to read the card to determine what, if anything, you want to do. On the other hand, you cannot take time during a fight to read every card on the table, contemplate your many (or few) options, or use a card to "discard dive," hoping to find something you can use in the fight. The "2.6 seconds" part of the rule is a joke; the intent is that you cannot hold up the game indefinitely just because someone else is winning, or, conversely, that you cannot immediately declare a win before your opponents can react.
Related Rule(s)