I think the thing that really made it feel so slow for me was that I had already seen
The Wrath of Khan,
The Search for Spock and
The Voyage Home before seeing
The Motion Picture, and although I understood the context of
TMP being the first time there was new live-action Star Trek content in a decade, I didn't really
appreciate that context.
Why did the fly-by scene revealing the refit
Enterprise last so long? Because it was the first time it had been seen in a decade, and it looked spectacular, and the filmmakers wanted to lovingly show it off.
Why did the fly-by entering V'ger's interior take so long, and mostly consist of back-and-forth shots of the special effects contrasted with the bridge crew, mouths agape? Because it was trying to establish scale that I didn't understand and appreciate watching a VHS copy on a 19" CRT TV at home. It was meant to be seen on a big screen. I couldn't tell that this:
Was supposed to be like THIS:
... so that I could see the teeny, tiny little
Enterprise flying by!