Windows' speed calculation is not always entirely accurate, particularly when used under a virtual machine. It doesn't mean that it's actually running at 5.33GHz, it's just that Windows' calculation is off - this can be due to any number of different factors. Either way, it's nothing to worry about.
The clockspeed of the CPU is variable, so at times it might display less than what it's actually running it. At you running the latest version of Parallels and have Parallels Tools installed on XP? I've never seen it run at anything less than expected speeds. Keep in mind that the current version just came out about a week ago. If you haven't updated since a while, you should do it now.
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