To be sure: is vaapi supposed to also help with encoding ?

$ pkg_info intel-vaapi-driver 

  The current video driver backend provides a bridge to the GEN GPUs
  through the packaging of buffers and commands to be sent to the i915
  driver for exercising both hardware and shader functionality for video
  decode, encode, and processing.

but ffmpeg -hwaccel is documented as

  Use hardware acceleration to decode the matching stream(s).

Jan