There are no ill consequences of using the fast mode, other than the fact that the radio timing is now different from earlier radio implementations (like the one in the nRF51 series).
As an example the ESB implementation uses the slow startup time to time the transmission of the acknowledge packet, and if an nRF52 device was sending an ACK to an nRF51 device using the fast startup the nRF51 device would not be able to receive it.
The power consumption will be the same. When using 'slow' mode on the nRF52 the radio is essentially just running a delay function under the hood, and delaying startup to match the older radios.
