Wow that's weird, I have no problems with my rMBP and have been super impressed with how fast it is with Final Cut and After Effects, two super intensive programs.
What Macbook are you using and how much RAM does it have?
What are the opinions on preowned macbooks? I was at a reseller looking today and they had a 2011 15 inch with an i7. Sounded like it was a top of the line one in 2011. It seems like the only thing I'd be missing out on with it is the retina screen, right?
I'm trying to do some reading but I'll be using it for graphic design and it seems like there's some varying opinion on whether you want a retina screen at all for that
I wouldn't trust unless it's through Apple or Amazon. If you go used through Amazon then ask for a serial number and if Applecare is on the device. Contact Apple and verify the serial number. I ask for multiple pictures as well.