Weird timing issue with Thread.sleep

[Andrej Mitrovic]

Take a look at this:

import std.stdio;
import core.thread;

void main()
{
    foreach (x; 0 .. 1000)
    {
        Thread.sleep(dur!("usecs")(999));
        writeln(x);
    }

    foreach (x; 0 .. 1000)
    {
        Thread.sleep(dur!("usecs")(1000));
        writeln(x);
    }
}

Compile and run it. The first foreach loop ends in an instant, while
the second one takes much much longer to finish, which is puzzling
since I've only increased the sleep while for a single microsecond.
What's going on?