What are some general things someone can do to boost overall performance in Processing?  

I have lowered my framerate and this is nice, for example

Here's a few:

Use multiplication instead of division (it's faster in most languages - look up why yourself)

Less objects. They make the code easier to understand but ultimately slow things down.

Use switch() instead of many if statements (look up why yourself)

float if you can instead of doubles, ints if you can instead of floats, bytes, etc. Less memory - less work for the PC.

my_variable++ instead of my_variable+=1

my_integer<<1 instead of my_integer*=2 (again, look these up)

ArrayList instead of using arraycopy again and again just to add one to an array (even better - just use a big array in the first place)

OPENGL

Turn off your other applications (and the screensaver)

Increase the maximum available memory in File>Preferences

Avoid unnecessary calculations (eg: you're working out if two circles overlap - so you figure you need to get the distance between the two circle's centers and compare that to the sum of the radiuses of the two circles. Why not just compare the square of the distance against the square of the sum of the two radiuses? You've just saved yourself from doing an expensive Math.sqrt operation to get that distance calculation. Again - you want to compare if two lines intersect - why not check the bounding boxes of the two lines first? You've just saved yourself from two division operations for a couple of lines that are nowhere near each other)

ermm...

There's more but's that's all off the top of my head for now.