Why do Android smartphones start lagging shortly after purchase?

Android doesn't process data related to User Interface as a priority like iOS. Instead it lets UI data process alongwith general data. (In technical jargons, the UI thread isn't processed as a priority). This leads to hiccups (lag) when normal data takes up more processing power and leaves little room for UI data to be processed and thus a delay in showing up the results of the input.

This is the prime reason for all that lag you face.

Another point related to Android's UI is that it is developed using JAVA.

The problem with using JAVA is that it is very resource intensive and has problems with the way it processes garbage values in Android while UI and apps run, therefore requires much larger processing power than the contemporary OSs like Windows or iOS.

This over time exhausts available RAM and thus slows down the phone.

This is the exact reason why iOS and Windows run super smooth on low RAM but you always see Android flagships carrying 3GB of RAM. Users think it is for them to use whereas the reality is that it's a fault of OS itself. It just can't run smooth without using all of that RAM. Larger RAM needs larger battery life too. That's why bigger batteries are found in Android handsets than in the other OS mobile phones.

All this just adds upto cost with no actual benefit to the user. Thus an Android Flagship costs significantly more than a Windows one. Pricing of iPhone is a different story. Their philosophy is to sell stones at the price of diamonds and make users feel special.

Yep! So the next time you see all those glossy advertisements smeared all over a newspaper's page telling you how big their battery is and how cool they are to offer you 3 Gigs of RAM, remember that much of that isn't for you to use. Android even after the recent Lollipop update just can't run efficiently on low specs.