If battery is the root of cause, you may verify it by a Watt meter.
Just checked mine; the Power Unlimited 10S 4900mAh 20C LiPo (40 flights) reads ~37.4V, ~67A, ~6000rpm at full throttle.
My other TP5300 10S 20C pack (153 flights) is ~36.2V, ~65A, ~5800rpm at full throttle.
These readings are just for reference since condition of my motor and test equipments may differ from yours. At least demo the idea how battery loading voltage affects the rpm. Hope this will help.

Luke