Yeah, I would guess if it was super slow, the most likely reason is a slow internet connection. Personally, I've never used the webinstaller. I know it's the recommended method, but I worry about exactly weird things like that happening and not knowing what's going on. Can you see what's going on with the webinstaller? Are there any kind of progress messages? I like the command line, because I feel more in control and I know what's happening. It also seperates the step of downloading the factory images (however long that takes) from the actual process of flashing, which does not take very long. Anyway, that's me. I was also already used to flashing factory images both from my CyanongenMod days and later from using Magisk to get root.