I’d love to know why people are downvoting you. You’re completely right - a minor update that would take 2 minutes on Windows or previous macOS versions now requires a 2 GB download and a 15-20 minutes install process.
I don't think the install duration is new. Most of the security updates on MacOS 10.15 have taken about 15-20 minutes to install on my 2020 MBP. And the first stage claims that it will only be a couple of minutes. Annoying as hell.