Tried it out, there is a definite decrease in meter gain after using ex moves. At the start of training mode, it seems the situation counts as having just used an ex move.
Exiting HEAT is not a contributing factor.
The decrease in meter gain is definitely time based, although getting more hits in shortens the time.
The amount of meter you have does not seem to affect your meter gain, however as I said, changing anything in training mode causes the decrease to occur again.
This is just five minutes of playing around in training mode though, I cannot confirm everything I've said is true.