I don't think there is time compression, but seems to be a 10% penalty on power duration on hardcore, and 20% on insanity (unless perhaps it just appears that way because the game is running 20% faster with no duration penalty).
Anyway, I just ran some tests on Adrenaline Rush duration, here's the results:
(Difficulty), (AR rank), (Passive), (Time):
Normal, AR rank 1, Combat Mastery 0: 4.3 seconds.
Veteran, AR rank 1, Combat Mastery 0: 4.3 seconds.
Hardcore, AR rank 1, Combat Mastery 0: 3.9 seconds.
Insanity, AR rank 1, Combat Mastery 0: 3.4 seconds.
Insanity, HAR, Combat Mastery 0: 3.4 seconds.
Insanity, HAR, Shock Trooper: 3.9 seconds.
Normal, HAR, Shock Trooper: 4.9 seconds.
These times were measured by recording video at 30 fps, then engaging AR while firing a Revenant LMG or watching squad mates move. Time represents the outer limits of the speed slow-down effect (including a 0.2~0.3 second transition at each end). For each condition I ran 3 or 4 trials, and averaged the results (spread was approximately ±0.05 s).
One thing I noticed is that there is also a 0.7 second delay AFTER time returns to normal before the cool down bars appear. This must be why the documentation reports the unmodified duration as 5 seconds (actually a 4.3 second duration + 0.7 sec delay). Omitting this delay, the numbers appear to correctly reflect a 20% insanity duration penalty and +15% Shock Trooper bonus.