mapping shared folders dramatically delays read/write processes

when i try to copy a folder with many small sized files to
a shared folder on the file server, the process takes very
little time, unless the same folder is also mapped to my
XP Pro computer with any drive letter...the mapping
produces a great delay in the whole process...Why?
i need to map shared folders to drive letters to work with
some applications...any suggestions?
i also used the share through the active directory with no
difference in the results...V.S.W.M. very slow when mapped