When playing along side friends, why is my phone always 30 seconds to 1 minute behind in detecting Pokemons?
Remember that this is a game that does not detect pokemon by some means of magic, but instead by some means of communication with Niantic's servers.
As far as I am aware, Niantic implemented this as an active call from your phone to Niantic's servers. This is a technique called "polling", where every time interval your phone will send a request (question) to Niantic's server with "give me everything near my position", and Niantic will respond with "well, there are these pokemons with these locations".
Every time they do this it consumes a tiny bit of data. Only finding the same pokemon 30 to 60 seconds after your friends is an acceptable trade-off for being able to use your phone data bundle for much longer, and for Niantic to keep the cost for their servers down. In other words: There is nothing wrong with your phone, or with your game. It's just how the game works.