well it doesn't, at least not on my Mac though it is not an iMac it is a G3 (466MHz/1M L2) i use xlr8's Mach Speed software to boost cache speed to 217.2MHz (that is what it reports)... my Mac crunches wu's at approx. 5.75 hrs on avg., without the cache acceleration it is a bit slower
this is just rough observational data, i have not felt the need to test things too closely with SETI because i am quite happy with the performance of my Mac running SETI