Word slow when opening a file on shared folder

Hello all,

I have upgraded a customer from Office 2007 to Office 365 Business. They have 3 PCs.

They all share documents from a shared folder on one PC (let's call it server).

When on a 'client' computer I access this shared folder and open an Excel file, but also an image or a PDF, it's very fast.

When I open a Word document, it takes up to 30 seconds to open, with the splash screen "Starting". 

Local Word files open smoothly.

I've tried the following:

1) Add the shared folder to the exception list of the antivirus (AVG) both on client and server

2) Delete the normal.dot both on client and server, and have Word create a new one

3) Check that all printers configuration are ok

4) Disable unused addons

5) Launch Word with the /a parameter

6) Create a new local file .docx (it opens fast), move it to the shared folder, open it (it takes a very long time to open)

7) If the file is already opened on another 'client', it opens fast because you can access it on 'read-only mode'

Any other suggestions?

Thank you very much,

Fabio