If I send some bitcoins to some address, and I pay the proper fees, how sure am I that it will be included in the next block? Are there any guarantees if I want my payment to be included in the next block that is mined?
Actually just pay recommend fees you can get your transaction sent less than 10minutes. Guaranteed
Your transaction will be relayed across the network in seconds for sure, but nope you can't be sure that it will be confirmed within 10 minutes.
As time between blocks is random, it is not uncommon to see no new blocks for half an hour or more.