@Neo33, thanks for the advice - though I can tell from the performance monitor that it was the CPU that was getting hammered, not the disks - and when I ran the performance tests, as expected, they showed everything was fine.
@cemarsh , I’d also done some more searching and testing myself… What I discovered was this.
I’m pretty much exclusively playing back the video via a 4K AppleTV. When I used MakeMKV to rip an 1080p HD DVD, it created a file that was roughly 5-6Gb in size, depending on the movie. If it was an old 720p SD DVD, then the file would be roughly 2Gb. Both of these files would have trouble when being streamed from the Plex server on the EX4100 to the Apple TV. On the other hand, if I ripped a Blu Ray, it would create a file around 16Gb but this file would have no problem streaming onto the Apple TV…
My theory is that the Apple TV is demanding 4K content, and therefore the Plex Server would attempt to send that. For the Blu Ray, the file is already in the right resolution, so it just reads the file and streams it. For the lower resolution files, it has to upscale them before streaming…
Now if only I can get the Apple TV to do the upscaling instead of the EX4100, then I think I’d be OK.
As a work-around, what I’ve been doing is using Movavi (a paid for product) to convert the .mkv files into .mp4 files instead - effectively converting them into files that are “4K Apple TV ready”. These then stream just fine.
The downside is that if I wanted to stream a film to a different device, that wasn’t 4K, I would probably run into the same processing problem as it needed to downscale… not actually tested that though yet!
Of course, watching an old 720p DVD on a 55 inch 4K telly (or even an 8 foot wide projector screen) really shows up the low resolution and tells me I never want to purchase anything but a blu ray again! Preferably a 4K one! Now if only iTunes would only sell everything in 4K! (And yes, I know that re-releasing a film as 4K is only possible if it was filmed with real film that can be rescanned at 4K resolution or if it was originally filmed in 4K digital format. Just wishful thinking! )