I used dolibarr everyday, for all the ERP out there this is the best so Far, we for some reason on Flexibility and At the same time ease for Auditing Time.
But the Problem is with MySQL because of connection disruption due to the slowness. If we can connect this to Mircosoft SQL this will be a game changer.
Difficult to answer, without knowing your current architecture and how dolibarr is installed.
What I can tell you is that even on a RaspberryPi3, Dolibar roxx !
MySQL is surely not the problem en slowness. This database gives better performance than Microsoft SQL.
The message you gave on the screenshot let think the trouble is a network problem or a trouble on your client (antivirus or plugin on browser that break the network connection). It might also be your proxy.
It is not related to the page you gave for “External access” (this page is setup when Dolibarr try to access external services and not when you try to access dolibarr. It is not use in common usage)