Conditionally populate cells in a column with a formula

For example, I have a dollar amount in column A and a currency in column B. If column B is any currency other than USD, I want column C to calculate something, but if the currency is USD, then no formula is needed. The only way I know to do this is to just write a formula that leaves column C blank if the currency is USD and paste it down to infinity. But is there a way to have excel selectively put the formula in based on my requirements, sort of like how conditional formatting will format cells based on certain requirements. My current solution works, but having that formula copied all the way down makes my workbook huge and slow.

Thanks in advance.

=IF(OR(I3="USD",I3=""),"",ROUND(O3/H3,4)&" USD/"&I3)

It's not the formula itself, just the fact that it's copied all the way down (~1million rows). It makes my workbook go from 500k to 10MB, and it's much slower now. Granted my computer is a little old but still, it ran just fine before but now everything takes forever.

Well no, I'm not using all 1 million rows (now), but I copied the formula all the way down to the bottom (1,048,576) so as I keep adding more rows, I won't have to ever worry about repasting it again. I suppose I could just paste it down a few thousand rows, but once I reach that last row, I'd have to paste it down again. It's just an inconvenience that would be nice to get around.