Comment # 20 on bug 911347 from Thomas Blume

(In reply to Bruno Pesavento from comment #17)
> (In reply to Thomas Blume from comment #16)
> > Hm, still 25s, not really good.
> > I'm currently looking at the upstream commits to improve journal flush
> > performance on btrfs (see comment#14).
> Agree, this is just a workaround to keep old systems rolling, waiting for
> the designers to solve the root cause. Journal logs are highly sparse, a
> 33.6MB file compresses to just 4MB in tar.gz, as a rough measure of info
> content...

Ok, still investigating more options.
A first and quick one is using the autodefrag mount option for /var/log.
For details, see: http://www.spinics.net/lists/linux-btrfs/msg41015.html.
Can you please check wheter mount /var/log with autodefrag has an impact?

Secondly, I've found another commit that could have an influence:
http://cgit.freedesktop.org/systemd/systemd/commit/src/journal/journal-file.c?id=eda4b58b50509dc8ad0428a46e20f6c5cf516d58

I will provide a new testpackages, but first please try the mount option.