Try to cheat decoding in ~/.xine/config:
video.processing.ffmpeg_choose_speed_over_accuracy:1
video.processing.ffmpeg_pp_quality:0
video.processing.ffmpeg_skip_loop_filter:all
video.processing.ffmpeg_thread_count:2
Interesting, it gave it a bit of a boost, although not a lot. Video
isn't as choppy as it used to be (actually, the video is now just as
good as on Windows without coreavc, which is still choppy) but the
audio is missing mostly... I think I'll have to give the other's
suggestion, the coreavc thing, a go, which isn't going without
problems, too: