Hold F2 while booting, to enter BIOS. Is the AC adapter recognized?

I have already changed performance mode to maximum performance in power category but still it does not helps and adapter gets recognised and charges the laptop without any problem. It's the system which drops it's performance when i plugged-in the charger.

Is the adapter recognized as a Dell adapter and is its power rating recognized within BIOS/Setup? Lower performance can be a symptom of a non-Dell power adapter or a fault with a Dell power adapter, or with the charging jack/port or motherboard charging circuit.

Check if the light is lit at the plug end of the AC adapter or on the adapter block. And see if that light remains lit when you plug it into the laptop's jack. If the AC adapter's light goes dark when you plug it into the laptop, then the fault is likely with the jack or the circuit.

Yes, the charging adapter is from Dell and I got it with laptop and the light stays on when turned on and while charging, the charging cable and adapter block seems fine.

Hello, StfuRohit You're charger should be at least 65 W To get the system to run at its maximum perfomance, check  you're charger first.

You can do the following,

• Check for drivers in supportassist especially for bios upgrades.
• Run battery diagnostics 
• Reset Bios To default

The charger indeed is of 65w, The power option in BIOS shows it is of 65w.

Ran the diagnostic on full system hardware, it shows warning on battery section and recommends to replace it, hope this stops the fps drops after replacing the battery.