ArcInfo GRID performance vs TIFF files

[Nuno Gil]

Dear listers,
I would like to ask if using GRID files or TIFF files as an input in
MapServer is the same in what concerns performance?
I have typically tiles of 5ºx5º degrees with 1 arcsec resolution
(approx 900 MB)... If I request a smaller part of that tile I suppose
MapServer does not need to read the entire tile, am I correct?
Regards,
Nuno