I can accomplish the same task using a database function (i.e. dsum), a
math function (i.e. sumif), and an array formula. If my goal is to
make the spreadsheet run as efficiently (fast) as possible, which
should I choose? Let's assume this is a very large set of data, and if
pertinent, the equation will be used frequently.

Also, I've heard array formulas in large volumes can slow down a
spreadsheet considerably. Why is this the case? What is the best
source for finding out information like this?

Thanks for your help.