Computer really lags while playing games?

I think you should overclock your video card and see if it improves.

1, don't overclock your GPU as suggested by other answer.

2, do you have Intel HD graphics aswell as the GT 6540m?

3, do you have windows 7/8/8.1?

4, run windows update (that is located in the start menu, all programs) and install the updates that are there. Once your version of windows is upto date, go to the manufacturer of your laptop's web page, find your laptop model and download the latest drivers for the laptop. If you don't know the model number, you may be able to find it with this:

Speccy
http://www.piriform.com/...com/speccy

it will either be in the system information page or some manufacturers mark the model number of laptops as the motherboard model, just google the motherboard model number.

5, install the drivers from Nvidia and if you have 'hybrid graphics' that are Intel HD graphics, download the latest off of intels site if they are not on or can't find the manufacturers page.

6, finally, defrag the drive,

Auslogics disk defrag
http://www.auslogics.com/en/software/disk-defrag/

do a defrag and optimize.

7* if you don't have antivirus or malware scanners, i highly recommend you do a full system scan with the following:

malwarebytes
http://www.malwarebytes.org/

if you don't have antivirus

Avast Free
http://www.avast.com/...n-gb/index

Edit_

forgot one thing, make sure your power profile/setting is not on power saver (click on the battery icon, or start menu, control panel, power options, OR type power options in the start menu search box)

set the power profile to either Balanced or high Performance (balanced is the best one, less heat produced and power consumed, laptop will switch between power saver and high performance when needed)

if you have anything scheduled or running in the background disable it. It might be the cause. Google 'how to disable' and then add 'Indexing' 'windows update' and if you have antivirus scans or defrag scans etc on a schedule disable them.

It really shouldn't lag when playing Minecraft and even Skyrim… Maybe you have too many processes running in the background at the same time. Check Task Manager by pressing ctrl+alt+delete on your keyboard and get Task Manager, and check all of the programs running. If this doesn't help, maybe its a problem with your computer's hardware (possibly broken?). To be honest I'm really not sure cause my laptop is much worse than that (8GB Ram, Intel HD Graphics 4000, Intel Core i3 2.4 Ghz, and 600GB HDD) and runs Skyrim on medium without lag with mods and I can play Minecraft with everything maxed out and still absolutely no problems. Sorry if I didn't help.

Just follow this tutorial
http://www.dmworldsoft.com/2014/01/how-to-drastically-increase-performance.html
It worked for me
NFS Rivals now runs smoothly on my laptop without any lags