You can simply set the preview resolution to half. third or quarter increments. The result of this is that the input resolution of source items is reduced, and the output rendered resolution is also reduced by the same factor. There would be no significant performance impact by reducing the actual resolution of a comp vs the previewed resolution.
Other performance enhancements during the design stage: turn off motion blur, switch to draft 3D mode.
Examine the flow of your layers. Anything that requires compositing before effects are applied can be precomposed or nested.
Create nests and render the nests out as self-contained movies. Import the movies. replace the nested layers with the movies and THEN apply the heavy duty effects.