@Joke Could you go to the Troubleshooting page on your DVR and run Re-Run with Resource Intensive Tests and submit diagnostics?
It could be that you have a failing disk, though with 4-6 recordings at once, it really could be that the random access read/write patterns are more than your Synology can handle.
The way the fingerprinting works is that it won't start a new fingerprinting task when there is any other activity on the DVR, which includes anyone watching, anything recording, etc. That said, if it is taking 11 minutes to generate a fingerprint of a file, if you started to watch something 1 minute after it started fingerprinting, you could experience 10 minutes of degraded performance (if you are at your limits for I/O).
Regarding the video indexing confusion: recordings will get a video index with Deep Video Processing turned off, but imports will not.