Idle Research: How to Level up Your Mastery Efficiently

Early on in the game, roughly after a few prestiges (took 3 for me) you will begin to earn Mystical Flasks upon each prestige. Once you have acquired one mystical flask, the option for mastery will be unlocked.

As the game is new we may have some math nerds decide the true optimal way to level in the future, but for now from my experience I have to found to level each flask to be the same level the easiest way to go, while still maintaining good growth in the numbers.
To accomplish this, continue prestiging to acquire more mystical flasks and also click the “change all” button on whichever flask you are attempting to level.
This will change all your flask crafting slots to be whichever you desire, increasing how much xp you gain for that flasks mastery slot drastically per second.