From Axe-Fx II Wiki
The information on this page supplements the official manuals.
A/D-conversion in Fractal Audio's products requires processing time and therefore causes some latency. Latency is the time between hitting a chord or note and the resulting sound from the unit.
The latency is very low and imperceptible.
(Axe-Fx II) "All the amps have the same "latency" and it is so low as to be imperceptible. The total system latency is about 1ms." source
(AX8) "The latency is the same as the Axe-Fx II." source
(AX8) "Latency is fixed and does not change with CPU usage. It's less than 2 ms." source