Unless Luther is airing with "limited commercials," BBC America is most likely cutting 14-15 minutes from each episode. In a standard 60-minute slot, BBC America cuts a show like Luther, which runs 58-59 minutes, down to about 44 minutes.
This is most likely due to low ratings. Luther S2 averaged just 125,000 viewers, which is very low for BBC America (as a point of reference, recent hits like Ripper Street and Orphan Black averaged 375,000 viewers). Being Human S5 was the same way -- the first 3 episodes aired in 75-minute slots at 10pm, but averaged just 100,000 viewers, so it was moved to a 60-minute slot at 11pm because repeats of Star Trek: TNG gets more viewers (average 200,000-250,000).
Broadchurch is averaging 500,000 viewers, so two weeks ago, BBC America announced that it would air a new Broadchurch this week and bump Luther to 9pm for one night.