This is hard to answer, because so much data is missing on your description. 
What exactly do you want to do?
What's your hardware (RPi version, HifiBerry version)?
Which input / output ports do you want to use?
Do you want to use a MIDI device connected to the RPi?
If the answer to last question is yes, then LE is not the right OS for you. Instead you'll need a real-time (RT) kernel OS (like Patchbox OS). That will provide low latency MIDI processing.
Read about advancedsettings.xml, chapter 2.8.5 cache -> buffermode. Switch off the buffer (mode 3) just to see whether it has effect.
Chapter 2.2.2 video -> latency could be the better option for you (proper video latency timing hopefully produces in-sync A/V).
Let it run over night, and hopefully the scraper did his magic on the next day. 
If I manually refresh a particular video the poster comes up but with over 700 movies it's a bit cumbersome.
Maybe it just takes time until everything has been updated automatically.
Thanks. Please try this advancedsettings.xml:
<advancedsettings>
<samba>
<statfiles>false</statfiles>
</samba>
<cache>
<buffermode>4</buffermode>
</cache>
</advancedsettings>Which LE hardware do you use? How do you access your movies (local storage, SMB...)?
This is issue because it turns on TV via HDMI-CEC.
You can avoid this at Settings -> System -> Input -> Peripherals -> CEC Adapter.
znaf: Same procedure like on config.txt.
When you read the doc, it says that cache settings can be relevant (2.8.5 cache -> buffermode). Please try this advancedsettings.xml:
<advancedsettings>
<samba>
<statfiles>false</statfiles>
</samba>
<cache>
<buffermode>4</buffermode>
</cache>
</advancedsettings>This should optimize SMB access for big file amount.
Yes, it is. I have some EGL experience, and I know there are some tweaks to speed up the GUI even without Vulkan. Feel free to start developing. It's open source, but EGL / Vulkan is no easy job. 
Usually those coolers install a script, which starts when the OS starts. On LE we have autostart.sh to trigger such things. You can adapt it.
It looks like HDMI audio extractors with an audio channel switch are critical: 6ch audio over HDMI audio trouble.
Unfortunately the thread starter Anke doesn't talk a lot, so I'm still unsure what happens. Maybe a different extractor solves it.
LE's GUI is coded in EGL, so in theory we could switch to Vulkan and get a faster GUI. In practice we need a Vulkan expert, who can adapt from EGL to Vulkan, and of course he / she has to do all the work for free - not very likely.
Sounds like a proof of my theory. Read the advancedsettings.xml doc and play around with cache settings. I don't think it's a bug.
The log doesn't shows related errors. Maybe you have to increase cache to avoid re-scanning. Try this at advancedsettings.xml:
<advancedsettings>
<cache>
<memorysize>139460608</memorysize>
<buffermode>1</buffermode>
<readfactor>20</readfactor>
</cache>
</advancedsettings>You talk about HDMI 5.1 audio. This doesn't exist, but now I have the idea that you eventually use an HDMI audio extractor with an 5.1 / 2.1 channel switch. Am I correct? A user at this thread currently has trouble with such an extractor.
If yes, then your described sound expansion board could work.