I've expected to compare a day period with same day period in year before. Two pre-archived reports. So I checked in the calendar-dropdown:
I expected to have two day period comparison like happend on «previous period» but worked like «custom», returning a range.
On huge websites, ranged report must be calculated on the fly by Matomo, while day, week, month, year periods are pre-calculated during cron archive.
The problem with ranged data is the server gets overloaded on big amounts of data. You can feel it by comparing «period year» with «previous year», then «period year» with «previous period». The second (pre archived) is faster.