[FFmpeg-user] libvpx-vp9 not using all 4 real cores (no HT CPU) like libx264 does (in top: ~250% vs ~387%)

[D]

Continuing and splitting up the problem which I have here: 
https://ffmpeg.org/pipermail/ffmpeg-user/2015-November/029274.html.
The problem is that libx264 utilizes all 4 cores almost to the 400% ($ 
top), as it should be, but when using libvpx-vp9, it's only ~250%.

$ cd ffmpeg-git-20151124-64bit-static

No Problem:
Almost 100% on all 4 cores if I use:
$ ./ffmpeg -i a.mp4 -c:v libx264 -crf 23 -c:a libvorbis b.mp4

Problem:
Only 1 core used, not all 4 cores like with libx264, but also on 100% if 
I use:
$ ./ffmpeg -i a.mp4 -c:v libvpx-vp9 -crf 23 -c:a libvorbis b.webm

Problem:
And trying to use all 4 cores, although all 4 cores are used now, it's 
only on ~60-75% utilization (~250% in top):
$ ./ffmpeg -i a.mp4 -c:v libvpx-vp9 -crf 23 -cpu-used 4 -threads 4 -c:a 
libvorbis b.webm
(changing  -cpu-used  and  -threads  higher than 4 values was neither 
faster nor slower).

What do you think? (See already mentioned suggestions in 029274.html 
thread. They unfortunately didn't help.)
John, the maintainer of the "Linux Static Builds" 
(https://ffmpeg.org/download.html), says it might be due to libvpx-vp9 
not being optimized yet and maybe someone else has an idea.
Can anyone try these libvpx-vp9 commands and say how many % in $ top are 
used?
My OS is Ubuntu 15.04 but I don't think this is the reason. My CPU has 
no HT support, only 4 real cores (84W TDP Haswell).