I'm not deep into the cryptocurrency world at all, but a good friend of mine is really into it and has given me a bit of Nano. Transactions are settled nearly instantly on a mobile device, and it relies more on a "proof of stake" than proof of work style system: https://docs.nano.org/whitepaper/english/

I really don't understand how it can be secure and haven't tried to read more into it due to skepticism about cryptocurrency generally, but are non proof of work schemes more viable?