Hi Bob,
Welcome to Capture One. You will find it to be an amazingly powerful package.
First, I think you were very brave to import your entire Aperture Library at once right of the bat. That said, Capture One is capable of handling collections larger than yours, so size is not the issue. I think what you are seeing is a lag in performance temporarily caused by background generation of previews. Because you imported 324GB of images, this will take quite a while to complete. Once done, you will see things improve considerably.
How much RAM does your computer have? Max it out to optimize your performance.
Have a look at the Free Rocky Nook e-book by Derrick Story for a guide on successful transitioning to Capture One from Aperture.
http://www.rockynook.com/making-the-move-to-capture-one-pro/
I have provided some other useful links in this other Forum thread:
http://www.dpreview.com/forums/post/57688583
BAD NEWS GOOD NEWS:
BAD NEWS: I've got about 85k images which are well over a terabyte. I was about to give up on the DAM when CO9 came along as, CO8 was choking to the point I couldn't move. CO9 gave me new life at the time, but I'm now struggling with speed again after adding a lot of images over the winter (yes, all previews are built).
Also, as my catalog grew, I started to find a lag when going from one of the edit tabs to the library/folder tab. This lag didn't exist before and is not horrible but it's getting worse. I'm on an i7 4-core Mac mini (late 2012) and 16GB. My solution will be to break my single giant catalog up into smaller ones. Which I haven't done yet.
Note: My catalogs are on SSD and my referenced image files are on a Thunderbolt spinning disk external drive. I highly recommend putting the catalog on internal or Thunderbolt SSD and using "referenced" file mode.
GOOD NEWS: DAM challenges aside, I'm a fan of CO9 editing, I just love it. The color editing , the styles and presets, the layer approach for selective edits, the RAW conversions. I also love the tethered shooting options (which many won't use but CO's implementation is the best as far as I can tell). I love the customizability of the interface and keyboard shortcuts. Unfortunately, to work that out, it took me a while to "get it", but now I'm quite nimble in the environment.
Note: I never "converted" any libraries. I just pointed CO at the same referenced library
folders and let it import EVERYTHING. Yeah, I lost the Aperture edits, but any TIFFs were fine and if I needed to go back and look at the Aperture version, I could. Forced me to learn the editing functions of CO. That said, I'm lucky because I always added KEYWORDS with Photomechanic
before importing images into Aperture. Thus, keywords were embedded in the RAW files and allowed me to import keywords into CO without doing the Aperture database conversion.
Bottomline: PhaseOne has got to improve the DAM. Telling people to buy Mac Pros is not an acceptable answer. My recommendation, if you don't have a 12-core machine, is to break up your catalog into chunks (I plan separate my sports photography into it's own catalog to start as it accounts for probably more than half my images).