Link is broken.

As for performance decrease, a few things can contribute to that:
1. Hard drive slow-down from clutter/degradation -- generally you'll notice longer time spent on load screens, games launching slower, etc. anything related to seeking and loading data from your hard drive.

2. Heat -- Dust can block airflow over time, fans fail or run more slowly, thermal paste dries up, or some other factor causing your CPU and/or GPU to run hotter, causing your hardware to throttle (decrease clock speed) to avoid overheating.

3. Games could undergo changes/updates that may put a performance hit on older hardware in favor of driver optimization for more modern hardware.

Most commonly it's the first factor, but the second one happens more often than people realize. The third one is more of a theory.

Anyway, right now, the definitive best $1200 gaming laptop is the MSI GT Series GT70 Dominator-895
NVIDIA GeForce GTX 870M 3GB
Intel Core i7 4800MQ (2.70GHz)
8GB Memory 1TB HDD
17.3" 1920 x 1080
Windows 8.1 64-Bit
DVD Super Multi

Now, your budget might be set in stone, and that's fine. But if not, here's what another $300 will get you: ASUS ROG G751 Series G751JT
Republic of Gamers
Intel Core i7 4710HQ (2.50GHz)
16GB Memory 1TB HDD
NVIDIA GeForce GTX 970M 3GB
17.3" 1920 x 1080
Windows 8.1 64-Bit
DL DVD±RW/CD-RW

It has double the RAM, which is not super important in terms of gaming, but of particular importance is the graphics card. It uses the GTX 970M, part of the latest generation of NVIDIA mobile cards, which has shown upwards of 40-50% performance improvement over the 870M (the graphics card in the $1200 laptop). When it comes to gaming laptops, every last bit of GPU performance is crucial, because it's essentially the sole dictator of gaming performance and among the most impractical and often impossible components to upgrade.

Having that said, I'm obligated to mention that if you don't necessarily require the mobility of a laptop, building a gaming desktop at that same budget would give you a significantly more powerful, easily upgradable machine.

This post was edited on 12/18/14 at 12:54 am