How long does it take to receive data from a cable, turn it into bits, then send it back out again?
High frequency trading? Video?

A lot depends on what the processing you need to do is. As soon as you need to buffer it that will insert latency. If you need to (eg) de-serialise it, examine two bytes, potentially replace them with a different two bytes, and re-serialise it, that will need a buffer of at least two bytes and realistically another byte for the cycle in which you do the processing plus a byte in each of the serdes implementations.

If you need to assemble a whole frame in order to apply the processing, then your latency is one frame per hop.

Buffering in the analog domain is hard, so this is only suitable if your processing is "cut-across" from one analog stream to another.

(Note that high speed fiber transcievers are expensive. There are 10G ethernet switches which have fiber input and output and an embedded low-latency FPGA for processing. I have one on my desk. It costs >$10,000 so we're only borrowing it)