[squarefoot]

It could very likely be Gnome. I would try accessing the same smb folders using a different desktop environment and file manager (eg. Thunar under XFCE, etc). The fastest one I'm aware of out there is Rox Filer which is already available on many distros and can be used in place of the stock one, but as far as I can tell it doesn't know anything about network shares, that is, it won't mount the share when the relevant mount point is accessed, the user has to provide an already mounted directory or implement automount. That's the price to pay for something that is like orders of magnitude faster than most used file managers.

https://github.com/rox-desktop/rox-filer