the HW architecture in the Axon is different than in UniPi 1.1 which you probably use as a reference. The ADC in the Axon is connected to the STM32 microcontroller which does the heavy lifting and the measured values are passed via SPI to the main CPU. The software protocol is modified Modbus, which is then translated to the ModbusTCP via unipitcp service. Therefor the fastest way is to talk to the unipitcp.
Since the communication is layered, there will be some slowing down along the way. Keep the requests to the other IO on the same group to as low as possible to speed up the communication. This would be harder to achieve in the Evok, which polls all the IOs all the time.