Because Wren's a bytecode interpreted dynamic language much of the overall time is probably spent just executing the surrounding benchmark code. I.e. in:

    FFI.start()
    while (x < count) {
        x = FFI.plusone(x)
    }
    FFI.stop()

It probably spends most of the total elapsed time on:

    FFI.start()
    while (x < count) {
        x = ...
    }
    FFI.stop()

It would be worth running a similar benchmark but with just the FFI part removed and then subtract that from the time. (And do the same thing for the other languages too, of course.)