Is my iMac faulty?

Hello, I bought a new iMac in the summer from a local mac authorized retailer. He transferred my backup from my old iMac (which had crashed) on to the new one for me - and almost immediately the new iMac was slow and clunky. So we decided to erase all the old data we had put on my new iMac and start fresh, bringing documents over from an alternative backup harddrive and downloading apps from App store as and when I needed them.

iPhoto had been the main issue on the old computer (crashing constantly etc), and again on the new one when we were working from my restored hard drive - even though I don't have an unusually large number of photos in iPhoto. To avoid it continuing to be an issue on the new install of the iMac we exported all the images from the old iPhoto and brought them into the new iPhoto manually, to avoid bringing over any "issues" that existed on the old one. The computer guy told me this would fix the issues I'd had with the new one - which were mainly that it wasn't really any faster than my old 2007 iMac.

Starting with a fresh install, initially the new iMac seemed a lot better - not blazing fast, but fine. However almost immediately I started having issues with the screen going black for a few seconds every now and then, but then it would come back fine. After a while this problem went away on its own, but now I feel the computer is excruciatingly slow for what is essentially a brand new iMac. iPhoto is again the main culprit, It can take 15 secs for iPhoto to load, and opening new windows in any program consistently takes 8-10 sec. I ran a disk speed test and it showed an average of 121MB/S write and 132.4 Read. I'm computer savvy, but not enough to know whether this is a really bad result or not.

Basically my question is, should I take the iMac in to an Apple store and ask for a new one or am I being overly sensitive to what are really minor issues (I have Apple Care and it's still under warranty).

The info under About My Mac is this:

iMac

27-inch, Mid 2011 (weird as I bought it custom built in June 2012??)

Processor: 3.4 GHz Intel Core i7

Memoer: 8 gb 1333 MHz DDR3

Graphics AMD Radeon HD 6970M 1024MB

2 tb Hard Drive

Running Mountain Lion