On my PC, The "substr(), y" method takes about 0.6 microseconds, the "substr(), s" method takes about 1 microsecond, the "unpack x$index" method takes about 2 microseconds.

The "unpack a$index" method takes about 41 microseconds (when averaged over a 262000 byte string), so it's the one to avoid. It has to allocate the large substring it skips over before it throws it away.