The card-side implementation of NFC payments is surprisingly resource hungry for parasitic powered NFC chip, but utterly negligible for a device with own batter

The card-side implementation of NFC payments is surprisingly resource hungry for parasitic powered NFC chip, but utterly negligible for a device with own battery, 64MHz ARM core and 4MB of storage. On the other hand good luck with getting such a device accepted by anyone in the financial industry. (On the other hand you probably could build something that proxies to your real physical card)