[AccessD] Terrible performance like I have never seen before

Hello AccessD Friends of Old,

I have a customer that is running an large-ish access app.  About 50 tables
and 10 forms.  Some large complex queries.

One a standalone machine it runs fast as lightening.  They say that in the
past it also ran fast on their network.  However, they say that since they
upgrade the backend and front end to Access 2007, the performance has
dropped through the floor.

I can see the performance and it is absolutely terrible 30 - 60 seconds to
open a form.  But now for the interesting bit.  It is only slow when the 26
users are using it !!!  yes, they have 26 users keying data all day long.  I
was gobsmacked because while I read about it, I never saw it happening in
real life.

To Summarise,
26 users - Access 2000 no problem
26 users - Access 2007 dreadfully slow
1 users - Access 2007 no problem.

This is pointing to a difference between Access 2007 and multi-user
situations but how can I resolve it?

I have prepared an mdb file in Access 2000 format for testing when the 26
people come into work in the morning, and if downgrading the BE fixes it,
that I am happy, but I do not think it is going to solve my problems.

Any idea?

thanks for your help

Mark