Niantic has increased the motivation decay rate for all Pokemon placed in gyms, including sub 3000 CP defenders. The official change log says the following:
Resolved a motivation decay bug impacting Pokémon with less than 3000 CP.
But we’ve observed a huge increase in motivation decay, roughly 1.35% – 1.5% every 10 minutes, resulting (on average) in a 10% motivation loss per hour. We’re unable to confirm, but early observations suggest that the decay is tied to the defender Combat Power.
Gym turnover will become much more frequent with this change, especially in the morning, when defenders are low and Trainers are asleep.
We’re not against this change, especially after the Remote Feeding feature has been enabled, however, gym difficulty has been significantly decreased.