A lot of flash player games run much faster when sound in them is turned off. First, you need to click mute in upper left corner of the application. This however does not helps too much by some reason. Probably because each game still have code that tries to run sounds on its own. So you need to turn the sound of in each game too.

Obviously, lowering graphics increases perfomance. A little. Each game have own graphics settings. In Aether, you can mark to have Low Quality starts. In Spewer you can turn off background decorations, and so on.

Very unobvious thing to do would be closing Steam window. I have noticed in many games that when Steam window is opened, there may be serious perfomance drop. No idea how is that ever possible. One of the games which have 50% perfomance drop when steam window is opened is Super Meat boy - another game from same author, so closing steam window may help this game too.

One thing to consider when trying to play with graphics settings is whenever you should run fullscreen or not. On my PC app window does not fit on screen in windowed mode, so turning fullscreen on decreases actual app size, increasing speed. If in windowed mode game is smaller, it should be left in windowed mode.

Still slow or lagging? Here is the last efforts.

Go into Windows resolution settings and select LOWEST possible resolution. Some resolutions only avaliable in 16 bit mode, so change that too. Also, make sure that screen is running at 60 Hz - most of games feel most comfortable at that setting.

When i changed settings from 1366*768 32-bit to 800*600 16-bit, graphic was slightly faster and input lag was much less. I was able to beat hard level in Spewer almost instantly how big changes were.