Slow links to files on network

[Robin]

I manage a set of files that collect budget information,
and have a report that dynamically links to a subset of
these files.
When a file is opened by another user, the creation of the
linked formulas becomes extremely slow, making the report
update almost unbearable (>2 mn for each file, creating a
hundred links or so, when normally it takes a few seconds
to do it).
Any clue?
Thanks
Robin