Thanks for your answer, MovieFan.Plex.
Something came to my mind just now: since the process is so slow, is it possible that, since I was triggering the process to an entire library, the progression was so slow because, despite indicating a specific TV Serie in the activity, it was considering the whole videos?
Now that the system was idle, I pressed “analyze” only on a single Dexter season, and I’ll let you know how it goes.
Update: after two hours it’s still at around 3%, so it’s just plain slow.
I put my hopes into Plex staff’s hands. :\
what is your synology model ?
i have 418 play with celeron bi at 2ghz (6gb ram) and it’s realy fast to analyse
what is cpu usage on synology when analysing ?
Hi Djantalu,
my Synology is a 718+ With a quad core Intel Celeron J3455 quad-core and RAM expanded to 10 GB.
Yeah, that’s why it’s really strange the analysis is so slow. It can even run a Windows 10 VM pretty decently.
At the moment it’s still analyzing the Dexter season I’ve started this afternoon (it’s around 28%) and the CPU load is minimal as usual:
Please forgive my jumping in.
10 GB of RAM with a J3455 will be problems.
The CPU is designed for 8GB only (2x 4GB).
Huh, the spec sheets posted above say only 6GB.
DS718+ is designed for small or growing businesses pursuing a compact and powerful
shared storage solution, offering the flexibility to expand the 2 GB RAM to up to 6 GB
RAM to process intensive workloads.
I wouldn’t expect that to slow down the system though.
Do you have this setting enabled?
Yes, the specifics say max 6 GB, but most of people using 718+ tried 10 or 16 and it didn’t give any problem. I’m using it since a pair of years with no issues.
Anyway I’m going to turn it off and remove the extra RAM for a test, just to cut out any potential issue.
No, it’s disabled.
Ok, removed the extra RAM, let the Synology restart and do all its checking tasks, started a TV serie season analysis and nothing changed.
Still super slow and still NAS CPU load at 1%. 
So it’s not something RAM related, going to add the RAM again.
Hi,
I, unfortunately, got the same problem after switching from a Truenas Core Jail installation to a Truenas scale docker installation. The server was completely rebuilt. My server is a Dell T410 with 128GB ram and two X5660. The only real change was that I installed an additional GPU (Nvidia T400) to speed up the transcoding. Before, the detection took a maximum of 10 minutes. Just as I wrote the message, I also found the fix for it I started an optimization of a random episode and pressed continue optimization and let it run. after that the intro recognition works no matter what series and episode.
New update:
I’ve made some tests with terminal “renice” command, just to see if, giving more priority to Plex processes, things could have been pushed to go a faster:
Nothing changed. So I guess it’s a Plex only problem, the Synology OS is not limiting it.
Any other idea?
i have a Synology DS1019+ (not sure if that has the same CPU as yours), but intro detection works perfectly fine for me.
New update: After Plex finally finished to sloooowly analyze the season I started yesterday, I gave a new try to @Tenster suggestion:
I optimized a single Grendizer episode, keeping it in original resolution (it was so quick I’m not sure it really did anything) but this kind of “unblocked” the “detect intro” function.
I’ve tested so far five or six seasons and it took like 5 minutes each, with NAS CPU that finally reached around 30% load.
I’ll spend a bit more time doing tests but maybe (touching wood) now everything is working as intended.
I’ll let you know the results in the next days. 
Hey sorry for the late feedback.
Glad to hear that it worked in the end but still the question is what the main reason for the behavior is. I’m curious what the Plex staff might find out because there are a few that it affects according to other forums.
As stated, intro detection is strictly a CPU dependent task that works best with Mpeg2 and h.264 files. Working with H.265 and unknown profiles will take much longer especially if you don’t follow the episode naming guide.
correct
Only if your HEVC/h.265 video file also has a complex audio track. Intro detection is primarily analyzing the audio stream
From https://support.plex.tv/articles/skip-content/
How to Analyze TV Shows to Find Intros
When enabled, your Plex Media Server will analyze the beginning of each episode, looking for segments that match between the episodes. This is primarily done by analyzing the audio of the file, which would find a common theme song, for instance. Matching is done per season, as many shows have a different intro segment for each season.
I understand what you mean @pl_5309, but problem is (was?) that I had a slow analysis on any file I tried. Now, after @Tenster suggestion, any video format (and, ofc, any of the previously problematic file) seems to work properly.
So there was for sure something that slowed the function in a big way.
Was the optimization paused when you started that? i.e. You had to hit “Resume” to get it going.
That was an old bug and should have been fixed a long time ago. Maybe it came back. 
Yes, I had to press “continue”.
Thanks, I’ll get that looked it.
