Is DS18B20 temperature sensor makes Arduino processing slow?
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.