I've had the MiBox S for a while now, and it works ok, what was curious if the streaming or buffering would be a lot faster on the Nvidia shield. More specifically, fast forwarding or rewinding tends to take a bit of time on the MiBox, even with a good 5ghz ubiquiti wireless setup. I know it's and expensive box for not running Plex server or playing games on, but I'm fine with getting it on sale if it would mean any faster and smoother Plex client (id be hard wiring the shield)
The responsiveness of Plex for fast forwarding doesn't really depend on the speed of your streaming device. It has much more to do with the performance of your server and the connection speed. Getting a good Ethernet adapter for your mibox would be about as beneficial if that's all you care about.
FFWD, RWD are dinosaur ways of navigating thru video. Time to reprogram yourself....
Kodi has - user programmable Skip steps that are far faster and more efficient to get to exactly where you want.
As already mentioned use Kodi and the PlexKodiconnect Addon.
The only thing then that really matters is Gigabit Ethernet if you are streaming high bitrate 4K files.
I FF without issue on 5Ghz using Kodi & my 2017 Shield Pro is on the other side of the house.
Just like Ferrari vs Ford
Fast forwarding sucks for me in Plex using Nvidia Shield using ethernet. If I hit it once or twice it is OK, but after a few times, it pretty much stops and if I play around with it a few times, it crashes.
If I use Kodi with the Plex addon, it's amazingly fast.