I do not know about Fortis but I can confirm that our customers really like DocuWare for its GUI, simple and efficient.
And in general, they like it because it is simple to use and they can get access to any kind of document from anywhere. The way everything is integrated in the same client, store, search, workflow tasks, viewer, ... is also a key element.
Talking about speed, I would recommend first to make some tests using the diagnostic mode available in the client. Tick the "Activate diagnostic mode" in Parameters/General and work with DocuWare.
It can show you if there is a bottleneck and if yes, where it is in the long process triggered to view a document.
The client PC performance is also important. SSD hard disk for the file server makes also a big difference as opening a document in DocuWare requires a bunch of I/O
Hope it helps