Are all the 2 amp Chargers are "Quick Chargers"? According to Qualcomm's website they use some modern technology which will result in quick charging and according to a blog on android police if you want to check a charger is a quick charger then check its ampere rating if it is 2 amp or not...So according to that all quick chargers are 2 amp chargers but is vice versa true?
Now why i am asking this? I have a xiaomi redmi 3s prime and i takes 3 hours to charge with a 2 amp charger(which came in the box) so is it because its battery is 4000 mah or is it because it doesn't support quick charging
All 2A chargers are not quick chargers and all quick chargers are not necessarily 2A (Qualcomm Quick Charge 3.0 is 3.2 V)


USB standard 3.0 in   lays down standards of 10 W: 5 V, 2 A  for smartphones and tablets 
Power = Voltage multiplied by Current and this equation determines charging speed. When you use a 2A charger instead of 1A charger, charging speed increases . You can
calculate the charging time required using the Battery Charge Time Calculator ( choose efficiency of charging at 90 % if your battery is not too old ). Your device is probably supplied 2A charger since the battery is big and charging with 1A would take long. You can plug in the values in the calculator and see for yourself
Quick charging / fast charging of entirely different.  Basic principle is to increase the power supplied in charging by increasing power, either by increasing voltage or current or both. See this for more details How does fast-charge work?