Comment by PhasmaFelis

This, yes. Also, if you're processing discrete items in any measurable way (downloading kilobytes, ingesting database rows, querying servers...) that takes longer than one second flat, display that to users. "Processing record 1/1,000,000... Processing record 2/1,000,000..." gives a positive sense of progress. Even if you don't actually know the final total in advance, you should still display whatever you've got.

As a bonus, this can occasionally be useful for bug squashing. "I've been waiting for hours and it's only up to row 5681" when you know for a fact that there's <2000 rows is a lot more informative than "My report won't finish."