The buffer size of your Sound Card, in simple words, this buffer is the small amount of memory that is used to store audio data in your stream, before it is processed by the digital to analog converters in your audio hardware and outputted though your speakers.
Usine calculate patches by "Blocs". For example every 128 samples (by default) which is equivalent to 3 millisecond.
Circular routings: rack 1 go in rack 2 and rack 2 go back in rack 1:
Usine add a latency of 1 bloc-size, but only for one of the rack, not the both. In all cases, the latency added will not exceed a bloc size.