Your swept area is greater in a crosswind which you've pulled forward with your bike's motion. With a dead-on headwind, you can get aero and reduce your cross section. But with an apparent wind forward of your beam, getting low doesn't do much. On our tandem, it's a real PITA because normally Stoker hides behind Captain and our aero drag is only about 50% greater than that of a single bike. But in a cross wind, we are both exposed and our drag is double that of a single rider. We hates it.
That said, I don't like any wind that's abeam or forward of our beam. Winds only from aft, please. Dead-on headwinds do decrease speed more than the same wind from the beam, however beam winds increase energy use because it's harder to control the bike and thus also increase danger. So it's all bad.