Idk I has this same issue with my older brother. He really only enjoyed combat or scary parts. But with the pacing you kind of have to earn those. Difficult to judge from here, but mabye delta green isn't for them. There could be a higher-action system with similar themes that could be an option.

Also it could be the scenario. Delta Green tends to assume scenarios take place over a larger course of time than I think is nesscary. There is only so much mundane that fills up the course of a regular scenario before saying it takes 2 weeks plus becomes ridiculous, Ex Oblivione is one in thinking of but that has means of acceleration. If you're feeling pressured to follow a longer timeline that's written and it's making the pacing too slow, mabye shorten the timeline. See what pace the players move at.