What should you do now?

Your choice to reduce speed is correct in this situation. According to German traffic rules on the Autobahn, when driving in the left lane (the overtaking lane), you must yield to faster vehicles approaching from behind by moving to the right as soon as it is safe. However, if the middle lane is currently occupied or it is unsafe to change lanes immediately, it is appropriate to reduce your speed slightly. This helps to avoid obstructing faster traffic and prevents dangerous situations caused by sudden lane changes. Once the middle lane is clear, you should move to it promptly to comply with the "Rechtsfahrgebot" (keep-right rule), which aims to keep the left lane free for overtaking only. This rule promotes smooth traffic flow and safety on German highways.