When the game starts slowing down, I just scroll to the bottom of the page and it speeds up. You can only do this when you are past having to actually shoot anything though.
Dual Mastery increases the stat bonuses that combined gems of different colors are given. For example, a Green an a Red gem combined would normally have roughly 1/2 of the bonus stats of a pure gem. As you level that skill up, the bonuses increase. Its the same with pure gem mastery.
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