I don't know if we had the 10% hit problem you're having, but I do remember a 32-bit 2005 instance that kept having log entries saying that it was under memory pressure. After researching it, we decided to upgrade to SP4 and that did actually solve the problem.

I was never able to figure out exactly what was causing the memory pressure. While I would have preferred to solve the problem instead of just installing SP4, I had my own pressure to "fix it" sooner rather than later.