CacheBuilder is obscenely expensive for concurrent map, and it requires generating unnecessary garbage even just to look up a key. Replace it with ConcurrentHashMap.
I identified this issue that impaired my own perf testing under load. The slowdown isn't just the sync. It's the expensive of Connection's ctor stalling other connections. The expensive of ConnectionId#equals causes delays. Synch'ing on connections causes unfair contention unlike a sync'ed method. Concurrency simply hides this.
BTW, guava Cache is heavyweight. Per local test, ConcurrentHashMap has better overal performance.