Good post by Sebastian Thaele (thank you)
Sometimes we notice that an ISL is actually a bottleneck in a fabric. Not a congestion bottleneck where the throughput demand is just too high for the ISL’s bandwidth. This one could be solved by putting another cable between the both switches. But if you have a latency bottleneck your ISL won’t be running at the maximum of the bandwidth. The contrary is the case: it lacks the buffer credits to ensure a proper utilization. If you see a latency bottleneck on an ISL it’s often back pressure from a slow drain device attached to the adjacent switch. But every now and then I get a case where it’s just the ISL. Sometimes in one direction, sometimes in both. Even with lengths were you don’t think about using long distance settings at all.
Read on here