If you are not redirecting to a file but letting it display to the terminal then a big variable is what GUI terminal emulator you are using - XTerm, rxvt-unicode, st, etc. I think that could create a lot of varying numbers..why it may even depend on if you use TrueType fonts or cheaper to render fonts.
Not sure what the goals are here, but if you really want to figure out what's going on at the core Nim IO layer then you want clean separation of sources of time spent (like input/split IO and output IO). E.g, forget about locking stdout buffers - just don't output at all. And on the output side forget about reading input - just generate the data like your generator and output and realize that if you are outputting to a terminal then you are benchmarking how that terminal handles extreme high update rates.
