I think sequences are designed for general purpose, not for ultimate speed. And due to the additional complexity (that the size can change), it cannot be as fast as arrays. For example, it has boundary checks. To perform such a check, the program has to first load its _current_ length and then compare it with the index. Arrays also have this check, but their lengths can't change. Try compiling with `--boundChecks:off`. Also try `--opt:speed --mm:orc`. Sometimes these options also matter.
As for `std::vector`, try using the C++ iterator (the internal pointer) instead of indices. Also `.unsafeAddr[]` is a no-op. It's just `*&` and will likely be optimized out, or decrease the speed if not.
