Capture NX2 on OSX 10.8.5
What I have is solid, but not quite a screamer. I shoot almost exclusively D800 36MP files.
I have a "Late 2012" Mac Mini with 4G of RAM and 1T 5400 rpm conventional drive. I have a 2.3 GHz quad-core i7. Its full specs are here:
I'm pushing a 30" monitor at 2560x1600 and a 24" monitor at 1920x1200.
All of my images are on an external 3T 7200 rpm drive connected through USB 3.0, backed up to another external 3T 7200 rpm connected through USB 3.0.
All of my wife's images are on an external 2T 7200 rpm drive connected through USB 3.0, backed up to another external 2T 7200 connected through USB 3.0.
These external drives are connected together through a Belkin 4 port USB hub that then takes up 1 USB 3.0 port on the Mac Mini. I sync/backup from Drive Files to Backup Files daily with SuperDuper. I should have an off-site back up as well, but... I just don't.
Once Capture NX2 is running, files open in about 7 seconds. I often open 10 or 16 at a time and then begin editing and putting them away one at a time, a habit born out of avoiding opening times from way back when I'd go get coffee while files opened. Now it's just a way to compartmentalize thought processes.
Most changes take perhaps 1 second to appear. It's not roughly real-time like Lightroom and Aperture generally do, but I don't have to import and export my files either. I do have a very occasional slow response that might take 4-6 seconds for the computer to catch up to the edits I'm doing blindly. But no major stalls.
I have no real stalls at all EXCEPT saving edited files. On my replaced Windows machine, (Core 2 Duo; 1.8 GHz; 4G of RAM) and similar drive setup, saving a file would take 40 seconds. On my Mac Mini, it takes maybe 10-18 seconds. I think that much of this is recompressing RAW files. After I tell it to save the file, nothing appears to happen for about 10 seconds, and then I get a quick write on the drive and it's done. I think I will start shooting uncompressed to see if that speeds it up further.
I just don't have enough experience with my new MacBook Pro, to make a comparison. I will say that I will not be using it to edit files on external drives, and it is a 512G SSD. It seems quicker so far, but writing files still would be the stalling issue. That, and the triangular menu buttons (to pull down the "camera settings" and "quick fix" menus), for instance.
I hope that's some indication of what it's reasonable to expect. Hope you get there. I do think that reinstalling Capture NX2 might be a great first step. You may also want to ensure that you have ample room on your hard drive for swap files. you probably do, but don't actually mention it.