What are the factors that impact the block time of a certain coin? Is a shorter block time always better or there's a trade off?
The block time is determined by each coins mining rules. In particular how difficulty is adjusted from time to time. Bitcoin adjusts the difficulty so that the expected time to mine is 10 minutes but a coin can adjust the difficulty to aim for any time.
Aiming for a very high time is inconvenient as users must wait longer to have a transaction included in a block. Using a short time increases the risk of orphans where two miners find the same block at once.