I don't think you're actually havnig any system troubles there, nor are you doing anything stupid.  Carrara (and most 3D apps) don't play animations at real speed, particularly when lots of polygons are involved.  Poser, of course, is optimized to move its models as quickly as possible, but even there, I find that I need to allow it to go into whatever it's called when you just see boxes in place of the figures.

Carrara will render your scene much more quickly than Poser will (and I think much better), but when you're working on the setup, it's going to be kind of slow.  If you're wanting to animate other objects inside Carrara after you've imported the animation from Poser, try either making the poser figure invisible or just showing bounding boxes.  I'm not in front of Carrara right now, but you can make the figure invisible in the properties panel.  There's also an option somewhere in the pull-down menus to make the object visible or not visible in the 3D views.  Just be sure to make the Poser Person visible again when you go to render!

Hope that's helpful.

 - Dex