X-noise is a VERY hoggish plugin and is not meant to be used as a real-time plugin. It is meant to be processed directly on the file. It says so in the manual. That having been said, I also see sluggishness with various plugins in CD Arch, especially with a lot of events.
I can't imagine using a plugin such as this in a CD writing program in the first place. Noise reduction is generally an interactive type process, and best done in an audio editor prior to thinking about CD burning, as settings can often vary throughout a track for best results.
But that's just me - that may well be your preferred way of working and I'm not about to suggest that your way should be the same as mine.
However it does seem that CDA5 has a higher processing overhead than, say, SForge when auditioning plugs though - it struggling even to preveiw O-Zone in realtime on my celeron 533, whereas SForge has no problem. Maybe some efficiency tweaking to be done...