Actually no. That's why this is highly recommended by a lot of people here. If you just look at the fact the 5-15 supplies 12A and the 5-20 supplies 16A, then you would think 33%, but there's a big factor that changes that by a lot. At these low power levels, the wasted overhead from just keeping the car awake and running the charging process is pretty huge!
I'm not sure how the Model 3 and Y are doing with this, but on the older Model S, this general constant overhead during charging was around 300-400 Watts. The 120V 12A from the 5-15 is only 1,440 Watts. That is about a fourth of it just being wasted, not going into the battery! So if you get to up the current to 16A, with the same overhead losses, that's all gravy added on top, going straight to charging, so it comes out noticeably quite a bit more than 33% faster.