I've seen a live performer in Stockholm, Sweden (where I live and I haven't had cash in my wallet for a few years, although I guess most people have some "just in case") accepting Swish [1], the instant mobile cash transfer solution (no fees).

I believe it is more about cash being more physical and thereby more immediately noticeable to the perfomer who can give the donator acknowledgement that is the major issue, not the technical transfer aspect.

I guess next step is to have performers carry a small LCD screen and an Arduino to instantly acknowledge received transfers to make it more "physical". (Wooah..! YC, here I come! (jk)).

[1] https://sv.wikipedia.org/wiki/Swish_(mobilapplikation) (Swedish)