Animating shapes vs masks in After Effects
Rendering a series of expanding circles (say less that 20) should not be a particularly slow task.

Check this page about how to improve performance in AE. http://helpx.adobe.com/after-effects/using/improve-performance.html

AE doesn't render the same way as non-linear editors like Final Cut or Premiere, so rendering can take longer (but users are usually working on shorter sequences than in FCP).

When you are preparing your work, try to work at lower quality settings, ie at 25% zoom, or quarter resolution, only zooming to 100% to check specific frames - get the motion working correctly at a lower res - this will save you a lot of time.

I don't think animating paths rather than masks will make a particular difference to the speed.