Network Test to support Real-Time Co-Authoring in Office

Hi, are there some network planning considerations for best performance of Real-Time Co-Authoring in Office suite?

We had a customer with really terrible and slow performance of coauthoring in Word on LAN connection. A lot of "keep server version" errors came out for users. But after switch to public internet connection or even local Wi-Fi everything working fine and fast.

We examine network and run classic connection and quality tests for other Office 365 services, but they seems be fine.