MacBook Pro slow with internal LCD, Fast on external

Hi all!
I'm new to mac-forums.com and also quite new to Mac. I consider myself tech savvy, but not really with Mac products so I was hoping someone can help...

The point:
I was given a MacBook Pro by someone who would like it fixed. They told me the notebook was going slow or sluggish whenever using the normal built in LCD screen, but once an external display was connected, it would run smoothly and efficiently.

They had another company look at it and they performed a full system factory restore and it sill had the same issue. They were stumped and gave it back. Now I have it.

Unfortunately, I don't have an adapter for the Mini-Display port to test an external display, but sure enough, while using the internal LCD, it runs very sluggishly.

ie. The mouse pointer is hesitant to move, almost like the mouse is on a reflective surface (but using the touchpad as well as USB mouse). Since the computer was recently restored, the Intro-Scene that displays after choosing a language is not smooth at all. It jumps and stops just like a streaming HD video on dial-up!

Any input on this would be greatly appreciated.

Thanks!

Let's start with some basic info that could help use solve this problem. There have been several variations of the MacBook Pro. Here's how to find out which one you have in your possession.

1. From the Apple menu in the upper left corner of the screen choose "About This Mac".
2. In the next sheet that appears choose "More Info . . ."

Jot down the info in the window that appears and post it here. This will give us some valuable information for troubleshooting.

Here's the data from that screen:

To add, the person that gave me this said one company replaced a broken screen, and it's been slow ever since. They sent it to an Apple certified repair shop and they ran diagnostics on it and said something was wrong with the display unit, but they couldn't do anything since it no longer had the original Apple screen.

Is it possible that when the screen was replaced, that there may be a connection problem? I'm very reluctant to open the LCD to check since I would have to break the seal and suction-cup the screen out..