(I added a serial.print (millis()); line to the beginning and end of both methods to figure out the time, if that is a valid way)
That mechanism for timing won't be perfectly precise but it is certainly "good enough"
I wonder, does adding the function bring overhead that can be reused if the function is used in other places?
Pretty much, yeah. Once the function is in, it is in, and you can use it other places at no additional cost.