There are 3 key features to MMR:
-Rating (skill level)
-Rating Deviation (confidence in rating)
-Volatility (consistency)

I am rather good at magic, i played a lot. Rating would be high.
I have been playing MTGA a lot. Deviation would be low.
I make a lot of custom decks. Volatility is high.

I would like to complain about my MMR. It doesn't move fast enough for my new decks. Not only is deviation not moving my MMR, the system updates in 24 hours to a week. Volatility is not high enough for my playstyle.

The only way i have to fix this, is to quit the game for a while to loosen my deviation.

I should not have to manipulate my MMR.

TLDR: Fix MMR Volatility.