Cutscenes use HQ assets as well as other effects, like DOF, which is likely why the framerate tanks in these modes. Character models feature more detailed textures than standard fights, and the background is blurred. You can also see this in photo mode, where the famerate too is comparable to the cutscenes/animations.
Doing a bit of testing, I'm suspicious that DOF is the culprit behind the framerate hits. Often the exit/entry fight animations, which make heavy use of DOF, have far worse framerates than many of the cutscenes in Chronicles mode, especially those using little or no DOF.
The system might have a bit of trouble rendering DOF in 3D, or it simply might require optimisation.