[lxgr]

> One problem with fast writing is that it requres more energy to transmit from reader to a card in order for a card that has no internal source of power, to toggle bits.

Contactless cards manage to boot up a JVM and run bytecode with the energy provided by the field, and many also run RSA with fairly large keys. I doubt that toggling some bits in an EEPROM factors in too much.

> I am not sure if single write is enough for making this robust/transactional.

Transactions are usually implemented via some form of two-phase commit, I believe, to support what's called "tear resistance". If the transaction is incomplete, the reader tells you and you just tap the same card at the same gate again.

Not sure about the implementation, but the feature is supported by all contactless stored-value cards I'm aware of.