Huh. That's not my experience. I usually watch like 8 gb rips of movies and I can watch them right away even though I pay for the cheapest speed my provider has which in practice is like 30 Mbit/s.
I have gigabit and I only download movies via Radarr and movies are automatically moved using a hard link from the download folder to the Plex folder on completion. And I'm never in a hurry anyways to watch a movie so I don't care.