The business records a high number of sales (or high value sales) and hence dirty money can be used as clean incoming money.

The high staff numbers indicate either a) staff can accumulate and no-one is really bothered because the normal economics don't apply, or b) more staff implies a bigger business (to anyone looking at the books), which means more money can be laundered without raising suspicion. Efficiency just depends on how much cash you can imply this coffee shop makes, so more staff is a good thing.