The point was rather that it makes parallel encoding more difficult. With a fixed interval it is trivial, you just split the incoming stream based on the fixed

The point was rather that it makes parallel encoding more difficult. With a fixed interval it is trivial, you just split the incoming stream based on the fixed rate, and let each thread work on separate intervals. With adaptive key frame placement you don't know the intervals up front, and they might have wildly different lengths. Some might be hundreds of frames, some might be just a few.