There are many many posts on DUC about denormalization.
Denormalization is how the processor handles (ridiculously) small floating point numbers (those not in "normal form" with a left shifted mantissa). It adds significant overhead to the processing. Most programs/programmers/users would be completely unaware of it. But some audio plugins end up with lots more CPU overhead when they see very low (typically no input or effectively no input) input signal. As I mentioned Dverb and Sansamp are historically well known for this. But many others can as well. you defeat this by deactivating the plugin, switch to a better behaving similar plugin, or give it a higher level signal to operate on--commonly by sticking a dither plugin in front of the offending plugin. And make sure you deactivate and don't just bypass any suspect plugins. Bypassing them can make things worse. ... here we are assuming the plugin is working basically OK just has some denomalization but not broken/grossly incompatible with stuff so you can flip thought sessions trying to deactivate suspect plugins to see if that affects CPU use.. but if you have hundreds of plugins or many instances of the same badly behaving plugin you may have to make lots of changes to see any affected. If you are looking for broken/incompatible plugins beyond a suspicion of denormalization you should play with removing/returning them from the plugin folders... that's the only way to know they are not messing up Pro Tools.
Some good hints you are seeing denormalization problems is the CPU going crazy when the transport is stopped... now whole lots of plugins might be getting no signal but the plugins are still running... and the CPU usage may go crazy. Other times you might see the CPU usage pulsate up and down. Or see CPU use zoom all the way up at some point when playing a session (like inputs to a plugin just went to zero). And at other times Pro Tools will just throw DAE/AAE CPU errors even without showing a high % CPU use... and denormalization can be a less obvious factor there, but so can other things.
Plugins seem to be better behaved and do this less in more recent software (that they ever suffered from this was really poor programming), but you are running old plugins on old Pro Tools.
While the CPU meters are giving you some help here don't over fixate on them. The real thing to make sure you get working is running heavier sessions without DAE CPU errors and the meters are not a perfect predictor there.
Denomalization is a suspect here so I'd be trying to see if I can prove it's a problem.
More technical explanations:
https://en.wikipedia.org/wiki/Denormal_number
https://software.intel.com/content/w...xceptions.html