Theoretical - Jitter from MESA Field I/Os

Hi guys,
maybe someone thought about this before. 
E.g. using a mesa card with a spindle encoder in a lathe configuration (use case: threading) - typically it's connected to the FPGA.
What would happen if the index-signal would come from a field io instead?
Imho the field io is not connected directly with the fpga but via serial communication, so I'd expect some latency btw. state change and the info arriving at the fpga. In addition I could think of some jitter coming from the implementation, depending on how it's is implemented.
Am I right? Didn't have time today to look deeper into the VHDL, but maybe you can help me find some sleep before I have to open the sources...

Regards,
Daniel
 

With Mesa cards, currently you cannot source an index signal from anywhere but the
FPGA encoder counter index input (because there is a intimate relationship between
the A,B and Index signals)

Ok maybe bad example - it’s more about the delays and possible jitter of the field iOS. I guess it will be a multiple times smaller than typical servo thread timers, so it does not matter…