1) one with Buffers is also causing mark-sweep/compact pauses (7-15ms each) because Buffer constructor calls AdjustAmountOfExternalAllocatedMemory which triggers full gc cycle if it thinks that too much memory is held outside V8.

2) GCs in string based benchmark are mainly scavenges taking 0 ms plus < 10 full collections taking <6ms each on my desktop.

That is all I can say. V8 GC is performing well here from my point of view.