Multiple issues

[serg]

DVD input works (you need just wait ~5 mins)

No clue why the indexing takes so long for you.
You might want to check that:
a. no anti virus or firewall solution is interfering
b. you source lies on a fast drive
Indexing requires to read the dvd once, but that shouldn't take 5min on modern hardware.

Got the files, will try to reproduce the issues this evening. (long work day today)

It looks like Avisynth still use ffmpeg for QT Prores (it takes  approx 5mins to open 20 mins file)

Other than Vapoursynth, Avisynth does use FFVideoSource by default.
-> Try whether enabling "Filtering->Avisynth->misc->Sourcefilter Settings->LibavVideoSoure instead of FFmpegSource" helps, when enabled Hybrid does use LibavVideoSoure  here.

Cu Selur

Enabling "Filtering->Avisynth->misc->Sourcefilter Settings->LibavVideoSoure instead of FFmpegSource" doesn't help.
It seems to me that the Avisynth  has an issue with QT Prores because Avisynth doesn't have LSMASHSource over LWLibAVSource option like Vapoursynth.
If in Vapursynth I disable "Prefer LSMASHSource over LWLibAVSource (that enabled by default), leaving "LibavVideoSoure instead of FFmpegSource" enabled, it behaves exactly as Avisynth (takes long time to load)