Laptop a tad slower than it should be?

[SachiNau]

My laptop has recently been... basically rebuilt after liquid damage. It's a Lenovo Y500, previously a pretty good laptop for gaming and 3D graphics, but since the new parts have been put in it's struggling with the most basic things, and I can't even run games on the lowest possible graphics without unbearable lag. It's been cleaned out, defragmented and run antivirus and antimalware multiple times, I've tried while disabling and closing all programs, and still there's that lag on low graphics. I feel like I'm out of options here. According to both GeForce Experience and System Requirements Detection I should be able to run most anything on Ultra not lagging on Low.
Specs:
Intel Core i7-3630QM CPU @ 2.40GHz
GeForce GT 650M (should be two of them but at the moment I don't know how to tell)
8 GB RAM
Windows 7 Ultimate

The CPU is used, is there any way to tell if that's the root of the issue? Any other suggestions?

EDIT: According to factory specs, there SHOULD be two GPUs but the device manager is only detecting one.
Drivers are all up to date
The reason why I'm asking about the CPU is that because before this one was installed, the service put in a Celeron (affectionately called The Potato) into the laptop as a placeholder so I could deal with some issues that required no significant computing power. With The Potato I was able
to run games on Low without lag, some could even go up to Medium. For some reason this i7 seems to perform way worse than the Celeron Potato.

 

[MMorris666]

Done a quick search, are you sure it has 2 gpu's? Check device manager to see if it is detected. If it is try updating your driver.

 

[SachiNau]

Done a quick search, are you sure it has 2 gpu's? Check device manager to see if it is detected. If it is try updating your driver.

It's supposed to have two, but it seems like it's only detecting one? Before the rebuild, it was very noticable that it had two, device manager showed both and the second one switched on when I ran something more demanding