Guild Wars 2

After being under the effects of a Karma booster for 55min, I have achieved diminishing returns! Is this normal? If so, why would I buy a 5 pack of boosters if I can’t maximize the effects for the duration of 1 booster?

Unless I’m amazing (which I doubt).

DR applies to the base karma reward (as it always does), not the booster. Unless you are presenting evidence to the contrary.

EDIT: On a second read, it sounds like you might be suggesting that the booster is not worthwhile because you hit the DR while under its influence. Of course since the DR applies in either case, the booster is still giving you additional points that you would otherwise not receive. So it depends on whether you are grinding for karma because you have the booster or whether you are grinding for karma anyway and want to get more out of it.