@darkstar2002 I use a sidecar processor on one board to support a Wiegand card reader. The processor is a ATmega328P (aka Arduino UNO) - the Photon then talks over I2C to the ATmega328P which tells it when it has read a card or keypad input.
It is possible to use a smaller/cheaper microcontroller, however, UNO is a great prototyping platform and there is space should you want to add other card protocols.
IMHO this would likely be the best solution with the Argon for the reasons @rickkas7 explained about the interrupt timing. For me the Photon has got enough going on managing the business logic and the user interface and cloud comms.