All iMacs since the ALS G5 have had 7200rpm drives in all configs. So either the drive was replaced with a much slower 5400rpm model later on, or the drive is dying.
This is, unfortunately, not true.
The late 2013 iMac 21.5-inch 2.9GHz unit in my possession, never opened and not a BTO/CTO variant, OEM-shipped with the HTS541010A9E662 — which is a
Hitachi Travelstar 5K1000, or 5400rpm on a 2.5-inch form factor HDD.