SUM is an aggregate function and ABS is a conversion function.
IIF is a conditional function, where at least two conditions have to be supplied and both must be evaluated and valid, even if only one condition is used.
By rule, conditional statements are slower than aggregates and conversions.
In looking at Roy's example again, it can be made just a few milliseconds faster by putting the conversion outside the aggregate, like so:
=ABS(SUM([10kforklift))
This way, the SUM aggregate function runs once, and the ABS runs once. Previously, the ABS was running for each value.
Keep in mind that unless you are working with millions of records (which I do), these little "speed up" tricks don't really have a noticeable effect. It's just good coding etiquette. However, I'm just anal about it that way. If you remember watching directory scrolls on a 386/12 to see how fast the processor was, then this makes sense to you.
~Moniker