Sure, this varies. If your POS system uses dialup, it can take tens of seconds, if it uses wifi it will be just a few seconds. If it uses contactless it can be sub-second. But in principle it can be as fast as the network and one trusted node.

The fundamental problem with bitcoin is that it is decentralised and trustless, and those two properties preclude fast transactions at a global scale - for that you need a federated system with trust between nodes. If you remove those properties, you're no longer using anything resembling bitcoin, and why not use a system with proper validated parties in the first place?