Fast VP9 still image encoding I use this command to create "myFile.webm", a VP9 video with one still image :

$ ffmpeg -i myOriginialPureAudioFile.opus -loop 1 -i myAudioFile_640x360.png -shortest -threads 4 -speed max -c:a copy myFile.webm
Input #0, ogg, from 'myOriginialPureAudioFile.opus':
  Duration: 01:53:19.80, start: 0.000000, bitrate: 24 kb/s
    Stream #0:0(eng): Audio: opus, 48000 Hz, mono, fltp
    Metadata:
      LANGUAGE        : eng
      TITLE           : unnamed
      ENCODER         : Lavf57.83.100
Input #1, png_pipe, from 'myAudioFile_640x360.png':
  Duration: N/A, bitrate: N/A
    Stream #1:0: Video: png, rgb24(pc), 640x360 [SAR 72:72 DAR 16:9], 25 fps, 25 tbr, 25 tbn, 25 tbc
File 'myFile.webm' already exists. Overwrite ? [y/N] y
Stream mapping:
  Stream #1:0 -&gt; #0:0 (png (native) -&gt; vp9 (libvpx-vp9))
  Stream #0:0 -&gt; #0:1 (copy)
Press [q] to stop, [?] for help
[libvpx-vp9 @ 0x5655578268c0] v1.3.0
Output #0, webm, to 'myFile.webm':
  Metadata:
    encoder         : Lavf57.83.100
    Stream #0:0: Video: vp9 (libvpx-vp9), yuv420p, 640x360 [SAR 1:1 DAR 16:9], q=-1--1, 200 kb/s, 25 fps, 1k tbn, 25 tbc
    Metadata:
      encoder         : Lavc57.107.100 libvpx-vp9
    Side data:
      cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: -1
    Stream #0:1(eng): Audio: opus, 48000 Hz, mono, fltp
    Metadata:
      LANGUAGE        : eng
      TITLE           : unnamed
      ENCODER         : Lavf57.83.100
frame= 4863 fps= 22 q=0.0 Lsize=     987kB time=00:03:14.48 bitrate=  41.6kbits/s speed=0.879x


The speed is about 0.88x, how can I make this encoding much faster ?
One solution (found here) is to use a very small -framerate value.