You can disable video conversion for specific types of files. This will drastically decrease the video conversion time. But you still have to use ffmpeg.exe to grab images from video files.

So, this is how to disable video conversion for specific types of video files (in my example, these are .mp4 and .webm files):

Remove the filters MP4 and WEBM this way:
using phpMyAdmin run the following query:
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DELETE FROM sys_transcoder_filters WHERE filter = 'Mp4' OR filter = 'Webm'
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This should be enough to stop video from conversion.
Clear the caches in Studio -> Dashboard

Of course, you will need to upload files only of the same formats MP4 and WEBM to keep videos playable.