Barriers are probably too small to cause latency spikes. The object logging barriers are a bit complicated, as they write an object into a thread-local log and infrequently have to get more memory for the log. But a card barrier is rather simple, and could look like

    SHR R_pointer, 9
    AND R_pointer, <cards>
    MOV BYTE [R_cards + R_pointer], 0

where <cards> is one below an immediate power of two number of cards, each card covering 2^9 = 512 bytes of heap. This is roughly the write barrier in SBCL on x86-64; there isn't much of a reason to think it will cause a latency spike.

On the other hand, "Barriers revisited" does mention a pathology about how cards can introduce false sharing, but that's still more of a smear than a spike.