Hi @SitalAgarwal, let me try to break your questions down to help you.
#Performance: You are buying into the ethos of Apple, others have done everything an iPhone can do before. Possibly a couple of years before. So what Apple does is take what others have done before and refine it beyond what others can even comprehend. This means your iPhone will be faster and smoother than other devices on the market. Not only that but it will feel better than others and most importantly, look better. Even Samsung have moved in the same direction as Apple with housed in batteries to make a uni-form device after years of promoting they allow a battery change.
#Restrictions: Back in the 70's Apple built their first computer using the Unix OS, even today the iPhones, iPads and Macs run on Unix. This means that the platform is a lot more flexible than that of say the Windows OS. The way the iPhone OS is designed means that the user parts are sandboxed. This means they operate in their own environment on terms stated by Apple. Believe it or not it is a lot cleaner than the way Windows works. It means that nothing gets in to any kind of Registry and nothing can take over the boot sectors etc like viruses or Malware. Even today malware has only existed on Jailbroken devices. However this does mean that you cannot install anything downloaded from external sources appwise. Ringtones can be installed from external sources using iTunes on a desktop/laptop computer.
#Security: Like previously stated, everything works in a sandbox, it cannot be broken out of without jailbreaking. Along with this the stock security from Apple is second to none. iMessage is an encrypted message service which even Apple do not hold the keys to. The American authorities are not happy with this as they cannot monitor communications. Apple Pay holds none of your details on any of the devices you input your card numbers etc on to so you are safe there. If you are US based then i think the health app works with your doctor, again Apple hold no records of what you record or share with the doctor/hospital. iCloud remains a big security issue with hackers as no one has managed to breach it since the iPhone 4(@MONGOLO is very interested in the iCloud bypass). Now if you lose your device or have it stolen with iCloud activated the device is useless as you can not only remote control it but find it using the "find my iphone" app and also remote wipe it if need be. This all adds up to a very secure device.
#Memory: The iPhone 5 comes with 1gb RAM inside it. Android phones do come with more power but thanks to Apple's unique task manager switcher your memory is freed up as much as possible at all times. Being as you cannot add anything to a start up task manager means the phone will always boot up as quick as it did the first time you turned it on.
The difference between the look and feel from a Nokia Lumia and an iPhone is immense. You will enjoy taking out your iPhone and checking emails and notifications a whole lot more than your Nokia Lumia right now. Get the iPhone 5 and enjoy being part of the Apple experience, you will not look back.