When working with high particle counts in Trapcode Form, the only thing that will speed things up is a lower number of particles. Creating an Expression Checkbox as an "on-off" switch for this is a efficient way to manage render times when working with Form.
Here is the expression used:
s = effect("Checkbox Control")("Checkbox");
if (s==1) value/4 else value
Make sure to add a checkbox control from the expression control effects.