Agreed. I did quite a bit of research in preparation of getting a new computer. Fusion runs better on a modest gaming machine than it will on a powerful cad (or business) machine. CPU speed is more important than cores and the video card is very, very important. Pretty much the opposite of how you would build a machine for AutoCad. See vtcnc's post (#2 here) for a link to system recommendations for various cad programs he just put up.When I first downloaded Fusion it would crash every time. After some research I discovered the problem was my video card, not my system. I swapped out the video card for a gaming card and now it runs perfectly. I think the video card is more important than people realize.
Slow internet could also be a problem. I think there is a way to work offline if you have your file downloaded. Try telling it to work offline.
Robert