Same here, firestorm and Harper both seem more sluggish than imprudence, and they use a lot more resources.
Last time I checked they were using around 50% of the machine's processors and ram if I remember right, and I have a 2 x 2 GHz Dual-core intel Xeon mac with 12 gb of ram. On a laptop I remember the viewer alone on it's lowest settings was using about 50% of that, viewers use a lot of system resources to begin with, but firestorm and harper both seemd to use more.
I have used imprudence 1.3.2 for a long time, it has no issues with high prim builds, I have a 9,000 + prim building on one region I built with it. It's been real stable and unlike the mac firestorm- it doesn't have that streaming music bug.