Here's a quick way to determine whether you have a software issue or a hardware issue.
Restart your system and boot into the Lion recovery mode by holding down the command-R keys when you power on the system. Once the system boots to the recovery mode, select shutdown from the Apple menu. If the system shuts down in the recovery mode then this is a good indication that your hardware is fine and that you need to evaluate your OS installation to see whether you installed anything recently that is causing the issue. It may also be helpful to disconnect any extra USB/Firewire devices that are attached so that you are working with just the basic system. It may be wise to reinstall Lion too.
1) It's slower than the other computer. I use them for Photo editing and put Lightroom and Photoshop Elements on it and the entire computer tends to lag at times when using them and the Internet. Granted I didn't have Lightroom on the first Mac, but still .. the lagging is something I would expect from a PC!
We have no idea what the specs are on either machine. The new line, came out in May of 2011, however, and according to the benchmarks, in the stock configurations, the lowest 21.5" from May 2011 outperforms the highest end 27" from 2010.
That being said, try creating a new user account and sign in to that. See how it performs. It may be a problem with your user profile.