HEVC Advanced Configuration?

[Cormy1]

I am using Constant Quality
So I was trying to get an idea of which preset I would like to use so I first ran an encoding with Advanced Configuration checked, and then with it uncheck on Very Fast preset.
The Very Fast preset encoding gave me a smaller filesize, with a shorter encoding time.
Isn't that completely backwards? Normally the difference between presets, as I understand it, is that you increase processing to reduce the filesize, so your encoding time should be longer.
So how is it that using the Very Fast preset yielded a smaller filesize with shorter encoding time than encoding with Advanced Configuration options off?

[eumagga0x2a]

I assume that https://avidemux.org/smif/index.php?msg=92739 answers your question.

[Cormy1]

Not quite but I suppose the implication is that it's purely a library behaviour and has nothing to do with Avidemux.

Is that also the case for the Stillimage tuning available in x264 that isn't available for HEVC?
Using that option I can get much smaller filesizes much faster than I would with HEVC's regular settings.

[eumagga0x2a]

Is that also the case for the Stillimage tuning available in x264 that isn't available for HEVC?

Exactly this, stillimage tuning is not available in the libx265 API: https://hg.videolan.org/x265/file/tip/source/x265.h#l2050 (if the line doesn't match anymore, just search for "x265_tune_names").

However, I can think of Avidemux being at least partially responsible for less than optimal compression with the default "advanced" x265 configuration as requested from the library by the x265 encoder plugin. The thing is that x265 defaults to "open GOP" type streams which are beneficial for compression rate and thus quality but break the ability to cut the stream in copy mode without damaged frames at cut points. For this reason Avidemux disables "open GOP" forcing the encoder to use proper IDR frames as keyframes.

If you are not interested in producing HEVC videos which you might easily edit in copy mode later, enable "open GOP" in the configuration dialog of the x265 encoder plugin. This might reduce file size of the output.