I am looking for suggestions - a synth kind of like Iris (have it and love it, but it's so CPU-heavy that it's not always possible to throw another instance of Iris into a project).  I've tried Dropzone...doesn't have that creative flexibility I'm really looking for.  Thoughts on a low CPU-impacting sampler?

I have not used Iris but based on description  it seems more like a synth that modifies samples, rather than a sampler.  Kontakt is somewhat of the standard when it comes to software samplers, but is nothing like Iris. With that in mind, you may want to try Alchemy. Not sure what the cpu hit is on it, but worth a look.

There are other sampler options but i skipped over them since most are more traditional in terms of sampler features than Iris. The free TX16Wx is a decent freebie. Geist is a sampling drum machine that actually samples, but neither have the sound mangling power of Iris.

Good point dubdisciple, it's more like a synth that manipulates samples that I'm looking for, ala Iris and Dropzone.  I'll check out that TX16Wx, looks like a contender...

Of the synths currently bundled with SONAR Producer, Rapture would be better at mangling a sample than Dropzone. The audio files can be loaded directly into any/all of the elements of Rapture or through sfz files.

Dimension pro has the same capability for SFZ as Rapture but has a bit different architecture. Both Rapture and Dimension load wave formats directly as well (wav, aif f e).
 

If Mach 5 is still available somewhere it could be nice I read. Or what about the Halion 5 workstation?

I got quite a bit of mileage out of this one when I was working on PC - I'd probably be using it if I didn't have a sampler built into my DAW.
 
http://www.discodsp.com/highlife/
 
 

There is also another free sampler called Zampler. There is lots of free samples for it on the web page.
http://www.zampler.de/

Hmm, some very interesting synths in this list.  Zampler is like a free Dimension Pro, kind of.  In all the years I've owned them (8 years?) I've never used Rapture or Dimension this way...the interfaces just don't lend themselves to sample manipulation in an intuitive way, all the sfz notepad-y typing, I looked into it when the synths came out but the learning curve seemed too steep.  I guess that's why I loved Iris so much when it came out, you can look at it and see what you're doing, for me a more fun and creative process.
 
I'll try out Rapture again, but the leaders from this list as far as what I'm looking for are TX16Wx and Geist, I think (I can't afford Geist right now, but it looks like a pretty capable synth).
 
Thanks for all the suggestions - you all put together a better list than any of the other forums I searched through with the same question...