profile.country.GB.title
I am not sure, but I thought Opus and Vorbis aren't supported by the TV. I know that Kodi is able to transcode the unsupported formats, since it plays a demo with DTS-HD (mute in any other player). And that would be CPU costly.
Anyway, I did a fast check with Kodi myself yesterday and a couple of videos did drop frames. I just went through my UHD HDR folder via DLNA. They all play smoothly in Video, though (unless there is a buffering due to network bandwidth. It seems in some points they may go above 100Mbps).
PS: YouTube version 2.0.18 has been leaked in apkmirror. But it is for arm64 only. I have setup the pushbullet notifications in apk mirror for the YouTube updates. That's how I know in real time. 
profile.country.GB.title
Ok, thanks! That has been quite informative.
Instead, back to the performances issue.. I have updated to the latest Plex version. Not touching the settings it does play all my HDR demos in HDR (I tested a dozen). And I have confirmatuon now that the high CPU usage is due to the.. downloading!!
Indeed playing the video while streaming the Plex CPU usage was at about 50-60%. Allowing the player to download the full video, and the play it, it went down to 10-15%.
I have posted the issue in the Plex forum, with all CPU monitoring. I wanted to add the link here but after editing it to add some tags, I have got the message that my post needs to be approved.
Actually I see I have it in the web cache (I mean, going back). This is what I have wrote (sorry for the ugly format but these forums are a pain):
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And it is still quite high even with 720p or 1080p videos. I used "busybox top" for the measurement in my Bravia 49XD8099. Plex version: 6.2.1.1036, the latest one in the Play Store.
When I play any UHD (HDR or less) video I get a similar CPU usage, with idle time always near 0%. Needless to say the videos lose frames, some even stutter:
Mem: 1294304K used, 319408K free, 89508K shrd, 11980K buff, 249128K cached
CPU: 55.9% usr 27.3% sys 2.7% nic 6.6% idle 0.1% io 0.0% irq 7.2% sirq
Load average: 41.58 40.05 39.43 13/2460 24302
PID PPID USER STAT VSZ %VSZ CPU %CPU COMMAND
20808 1242 u0_a169 R 1987m125.7 0 57.8 {plexapp.android} com.plexapp.android
1233 1 system R < 155m 9.8 0 9.2 /system/bin/surfaceflinger
1265 1 media S 129m 8.2 3 4.6 /system/bin/mediaserver
1085 1 logd S N 17956 1.1 1 2.7 /system/bin/logd
13720 2 root DW 0 0.0 2 2.0 [SonySOPQCtrlVdo]
Pausing the video I still get quite an high CPU usage (Plex should be just reading the video ahead):
Mem: 1454824K used, 158888K free, 88800K shrd, 6872K buff, 256992K cached
CPU: 43.6% usr 26.8% sys 2.3% nic 19.1% idle 0.0% io 0.0% irq 7.9% sirq
Load average: 40.03 40.11 39.52 3/2446 24469
PID PPID USER STAT VSZ %VSZ CPU %CPU COMMAND
20808 1242 u0_a169 R 2170m137.3 0 46.8 {plexapp.android} com.plexapp.android
All these videos just played fine with version 5.10.x.y. Buffering apart (that would be another issue. Often it doesn't use the full LAN bandwidth - 100Mbps). I decided to update (also for testing) and I have seen that updating it keeps my previous settings, and it doesn't trigger the 4K transcoding bug. So unless it breaks again I'll keep it as it is.
Actually, no. It IS due to the downloading! If I pause the video waiting until it is all in memory, while playing the video I get this other CPU usage:
Mem: 1440380K used, 173332K free, 91200K shrd, 4376K buff, 234412K cached
CPU: 14.1% usr 16.7% sys 0.3% nic 68.7% idle 0.0% io 0.0% irq 0.0% sirq
Load average: 38.39 37.41 38.35 3/2508 25436
PID PPID USER STAT VSZ %VSZ CPU %CPU COMMAND
20808 1242 u0_a169 S 2148m135.9 2 10.0 {plexapp.android} com.plexapp.android
So what is it, to download 8-9MByte/s sits down the CPU?
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