vn(4) performance on 4.11 versus md(4) on 5.4

[Glenn Dawson]

(I sent this to freebsd-performance yesterday, but that list seems kind of 
dead, so I'm trying this one.  If there's a better place for me to send 
this, let me know.)

I have a number of systems running 4.11 that have file backed virtual 
disks, each of which contains a jail.  I need to start using 5.4 for new 
servers.  The catch is, file backed virtual disks using md(4) seem to be 
much slower than similar virtual disks on 4.11 using vn(4).  vn(4) on 4.11 
is about 2.24 times faster than the equivalent setup using md(4) on 5.4.

I've posted the results of some tests that I ran at 
http://www.antimatter.net/md-versus-vn.txt

Is this decrease in performance known?  Is there something I can do in 
order to come close to the performance that 4.11 has?  I've tried changing 
some of the parameters of the filesystem on the virtual disk, but the 
performance didn't change.

I can supply more info than what the link above has, if needed.

-Glenn