Nvidia Shield vs Roke Premiere+

So as I understand, the Nvidia Shield is the best experience you can get atm for a standalone Plex Client, but I am a little over budget so I have some questions before I make the investment. A little backstory first:

For the longest time I was using a hardwired Chromecast to play 1080p content. However it was slow and really bothered me because subs/dts really slowed it down. I have just upgraded to a high-end 4k TV (Sony 900f) and I want to make the best of it. I knew I would eventually have to upgrade my receiver (from ~2012 with 4K passthrough) to make full use of HDR but I didnt realize the plex on the TV was unable to passthrough DTS. So I got a new receiver and alongside the Roku Premiere+. My issue with it is that its slow at playing large files. There are times where it even tells the network isnt fast enough to handle the content (was watching the new Thor BUT it was a REMUX and to be honest I cant tell the diff between 30gb vs 50gb files so far, so I don’t actually expect this to happen alot).

So for my question, if I am going to spending an extra 150 CAD, is the Shield going to be really fast? I mean a second or 2 tops for seeking between files? Or should I be looking at 3rd party stuff like Minix or other DIY boxes?