It seems in several applications Vaapi significantly worsens performance.
CPU-usage:
VLC without VAAPI: ~40%
VLC with VAAPI: ~90%
MPV however seems to work fine:
--vo=opengl --hwdec=vaapi: ~8%
--vo=vaapi --hwdec=vaapi: ~8%
--vo=vaapi --hwdec=vaapi-copy: ~16%
--vo=opengl --hwdec=vaapi: ~50%