Sluggish 2011 iMac query

Hi

I purchased my first Mac, a 2011 27" iMac about 9 months ago. One of the first things I noticed was that relatively small apps such as Mail, Calendar, App Store, etc. would take a few seconds (about 2-5) to open. I assumed that this was normal for a Mac until a few weeks ago when I used an entry level Macbook Pro 13" and noticed how snappy it was. Another thing I noticed was that my iMac's hard drive would constantly be making a noise when opening an app, browsing the web, using finder, etc. Then a few days ago my mission control animations started slowing down. I tried everything to fix it including PRAM reset, disk permissions, etc. Nothing worked and it annoyed me immensely so I decided to erase the OSX partition and reinstall Lion from internet recovery. After doing that mission control was fixed but strangely, the entire machine ran A LOT faster. Opening Mail now only takes a split second. Browsing the web doesn't make the hard drive go crazy anymore. And I don't have to wait 10 seconds for an MP3 folder to open.

So my question is, what could have possibly caused the iMac to struggle from the very first day? I would like to find out in case it's something that could happen again so that I may avoid it. The hard drive is a western digital, not one of the seagate hard drives that would have been recalled. I know it's a strange question but any advice would be appreciated. Could Lion have been installed incorrectly at the factory?

Though it's not very common to see a Mac behave in the manner you describe, it is possible I suppose depending on how the HD's were made. My iMac came with Snow Leopard and was snappy from day one, I then upgraded that to Lion and it continued to be just as snappy and now I've upgraded to Mountain Lion with no perceptible reduction in performance.

But glad that you've gotten the system working as quick as it should. It's hard to figure out what happened in the past, but with ongoing maintenance you can ensure that it continues to work as well as it is. So keeping enough free HD space, running OnyX on occasions and even Repair Permissions to keep things in check should be good..