The block heater should have zero effect on how quickly your coolant warms up (ie, the temp needle will continue on its normal pace). To Dreamliner's point, its sole purpose is to heat the oil so it circulates fast and isn't sitting in the block like molasses. The block heater is a means to avoid accelerated wear in extreme cold weather starts. What you should notice is in extreme cold (-20F or below), the motor doesn't sound like it's trying to destroy itself when you first start it up. With the exception of the slow cranking from a cold battery, if the block heater is working correctly, the motor should sound 'normal' when it turns over rather than grinding metal on metal for a few seconds.
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