Elam explains that at this point the team feels that the core game mechanics are where they need to be. What the game does need is to trim out bits of visual fluff and effects that force the game to run more slowly than it should. So the developers are working on simple ways to do that, like slowing the rate of updates on adjacent players to speed up large battles and simplifying the physics engine to not render changes that players cannot see or interact with. For a full list of the optimization projects, take a look at the letter.
[Thanks to Gustavo for the tip!]