Plex/Emby/Kodi all do something similiar to a ffprobe/mediainfo command to get the codecs and such for the file. I’m not as familiar with Kodi as I’ve never really used for more than a few minutes.
Chunked or partial reading means that is does a request for a piece of the file at a time. If it’s set too big, you can get some waste, but Plex closes the files so fast, it really is insignificant in terms of the initial library scan. File size semi matters but usually what’s in the container dictates how long it takes when it’s scanned.
I usually see anywhere from 2-10 seconds per file depending on the file.
buffer-size would help if it was bigger in terms of direct playing or a process using the file in a sequential fashion. For Plex, this means Direct Play. I personally just it at the default value for the buffer and I never have seen an issue with it.
Having a large buffer size means if a bunch of files open, you potentially can run out of memory for the system.
My general is approach is to keep it simple and leave everything at defaults unless I have a very specific reason to change it.