Rembrandt [Radeon 680M]: excessive power consumption when decoding AV1

This is on Ryzen 7 PRO 6850U, Advanced Micro Devices, Inc. [AMD/ATI] Rembrandt [Radeon 680M] (rev d4). Using kernel 6.6.1, mesa 23.2.1 with firmware

[drm] Loading DMUB firmware via PSP: version=0x0400003F              
[drm] Found VCN firmware Version ENC: 1.27 DEC: 2 VEP: 0 Revision: 0 

Sample video file:CityHall_1920x1080

Running

while true; do
  ffmpeg -v verbose -hwaccel vaapi -hwaccel_device /dev/dri/renderD128 -i CityHall_1920x1080.webm -f null -
done

while on battery increases power consumption from ~9W (idle) to ~19W (decoding loop). Also the fans are starting to spin.