[raluk]

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. It is harder compared to just reading a bit from a card. Additonaly it is tricky to implement trasaction with single write, given that data transfer can be interrupted (for example user removes card from RF field). I am not sure if single write is enough for making this robust/transactional. It also helps a lot if RFID antenna is well tuned. Proximity of metal and way it is mounted has a big impact, so it is important that RF antenna for reader is tuned for exact environment it is mounted in.

[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.