First of all, thank you for your feedback !
I am afraid that you read a part of the article somewhat too quickly. Indeed, I wrote that FileVault does not lead to a noticeable slowdown in a real-world business environment. Users who rely on resource-intensive video or audio applications, should take extra steps, as stated.
I am afraid that your perception of the speed of Mac OS X does not reflect its real performance. Indeed, many studies have shown that its UNIX core is very fast ! You will find numerous links to such reports on the Apple site as well as on independent third-party publications.
Mac OS X or its UNIX base alone, Darwin, are now used my many research labs to perform some advanced calculations that require stability and speed, as well as by some of the leading computer animation companies that require a OS and hardware capable of performing intense calculations.
Again, thanks for your feedback.