"I want a cheap and fast system for small/instant transactions in my physical shop!!!"
"Ok, the rational choice is to accept credit cards, applepay and stuff like that."
"Ok, the rational choice then is to use trusted third parties for bitcoin-nominated tabs."
"Ok, the rational choice then is to use trustless layer 2 systems."
"Ok, the rational choice then is to invest some money and developer hours in building those solutions."
"You're asking a lot. Not free/instant, but you can start optimizing onchain blockspace use: SegWit, batching, good coinselection, good fee estimation w/ atoRBF."
"Not sure you can have that, my friend. Sorry."