Calculating Unique users takes more resources than everything else because it has to compare more data with each other.
This doesn’t matter for e.g. weekly reports as the cronjob (if you have set one up) will calculate it beforehand and one doesn’t have to wait. But date-range reports have to be calculated on the fly, so it would slow it down.
If you need those reports or your server is fast enough, you can enable it as shown in the FAQ above.