Excel slow!

Dear Reader,

Hope everybody's fine. Great to be here. I work as an IT Consultant and some
of my clients are having problems with excel, at a sudden excel is getting
slow and slow without an apparent reason, calculations and navigation takes
longer than usual on different and fast computers.

However when they are not on the network it gets a bit fast than when on it.
Could you please help me solve this problem, I have tried install and
unisnstall but nathing changed.

Hope to read you soon.

Regards,

Olivier