My daily driver iMac is an i5 model from 2009 (2.66GHz, 16GB, etc.) I use it 8-10 hours a day, five days a week, as a high-end print designer. Think InDesign files that are hundreds of megabytes in size, along with all the giant Photoshop files, complex Illustrator images, etc.
The limitations I run into are more about memory—even the comparatively ancient i5 I have is more than enough horsepower for a heavy-duty workflow. If you're buying a new Mac now, I would worry less about the processor since they're all going to be "fast enough" for almost all uses, and concentrate on the other systems, particular the internal storage and memory, to make sure they are up to what you intend to use the machine for.