Github user ben-manes commented on the issue: https://github.com/apache/metron/pull/940 Guava defaults to a `concurrencyLevel` of 4, given its age and a desire to not abuse memory in low concurrent situations. You probably want to increase it to 64 in a heavy workload, which has a ~4x throughput gain on reads. It won't scale much higher, since it has internal bottlenecks and I could never get patches reviewed to fix those. I've only noticed overall throughput be based on threads, and never realized there was a capacity constraint to its performance. One should expect some due to the older hash table design resulting in more collisions, whereas CHMv8 does much better there. Still, I would have expected it to even out enough unless have a bad hash code?