Liquid Mix 16 Issues

[Joesi]

Perhaps someone of you can help me:
I just bought this Focusrite box, installed it and it works. So far so good. Though there is something that confuses me. The LM is an external DSP, right? So why does Reaper report a CPU usage in my FX window once the LM plugin has been loaded?? I have an old P4 with 2.6 GHz and 2 gig RAM and reaper says that this single Liquid Mix Instance uses up to 14 % CPU??? If this is true, what do I need an external signal processor for? Do you know this issue and what can I do to solve that problem? Do I have to change some settings in my prefs?