Ships with gas turbines tend to run more efficiently at 30+ knots, and carriers tend to conduct operations at about 30 knots with reserve speeds being even higher.
In relation to ships like 075 (other LHDs, LPDs etc) they only tend to be able to do 20+ish knots as their top speed. The Hyuga and Izumo classes are the exception.
I think it's more accurate to say that gas turbines run more efficient at full output.
But much of the time, ships like the Arleigh Burkes only have 1 gas turbine in operation, with the other 3 switched off.