Is DS18B20 temperature sensor makes Arduino processing slow? I recently run into a problem that causing my whole code processing slow down, for example i had a millis timer set for one minute but it took 1:30m to finish, pressing buttons was laggy etc. 

After hours of digging and commenting out codes piece by piece i finally found out what is causing the slow down!

my DS18B20 temperature sensor DATA pin is connected to pin A0 when i remove the library and related codes to the sensor processing became MUCH faster more snappier/responsive...

After googling a bit i realized that everyone connected the sensor to pin 2 of Arduino!

So is the processing slow because DS18B20 is on pin A0 instead of pin 2 or there's something wrong with the sensor itself or it is because of This library?
Indirectly it's the library.

That library uses the OneWire.h library.  The OneWire protocol requires some reasonably precise timing, and because of the the OneWire.h library turns off interrupts while it communicates.

While interrupts are disabled nothing else can happen. That means that the timer interrupt that drives millis() can't "tick" - so the millis count doesn't increase while you communicate with the temperature sensor.