delay(); will delay at least the amount of ms speciefied by the argument.
You are not guaranteed a delay of precisely the requested amount.
[edit]Of cource, the code between the millis(); calls will have an impact on the frequency of the calls, but won't impact the millis(); call itself.
And when I think of it, I am uncertain of my previous statement. Maybe you are guaranteed a millisecond resolution, but not, if I remember correctly microsecond resolution.[/edit]