Just did a quick test in Excel 2010.

Average calc times for these 2 formulas:

=SUMPRODUCT((A1:A100=B1:B100)*(A1:A100<>""))

=SUMPRODUCT((A:A=B:B)*(A:A<>""))

Did 5 calculations and averaged those results.

Formula1: 0.000842
Formula2: 0.121852

Time is in seconds

As you can see Formula1 is significantly faster to calculate compared to Formula2.