People have been reporting that the Magsafe charging is tremendously slow, unless you specifically have the paired 20w charger.

I heard the same. Then I saw this video, and now I'm not so sure.

MagSafe: Does it charge at 15 watts? MagSafe test with 3rd party PD power adaptors

https://www.youtube.com/watch?v=yDbcrbZdKGI

Are you sure? Those chargers are 30W and 60W.

I don't know firsthand, but these guys say it's true:

https://appleinsider.com/articles/20/10/27/magsafe-15w-fast-...

Apparently the adapter needs to support the 9V 2.2A profile, which is uncommon. So the max wattage isn't all that relevant.