We get quite a few questions on why CPU usage is below 100% in some operations. This applies to both PrE and PrPro. Here is a THREAD (on PrPro, so there are some Pro-specific comments involved in the thread), and Harm Millaard has distilled the possibilities nicely in Reply #1.
Note: there is a link to the PrPro CS5 Benchmark. The site, and benchmark files and tests are not available for PrE, but when comparing hardware, the spreadsheet of the performance data from users' configurations, can tell one a lot, even if they are running PrE, and cannot actually participate in the benchmarking - the data in that spreadsheet is still VERY useful, when looking at various computer configurations.
Good luck, and hope that this helps others,