NFSv4 performance degradation with 12.0-CURRENT client

I have a FreeBSD 10.3-RELEASE-p12 server exporting its home
directories over both NFSv3 and NFSv4.  I have a TrueOS client (based
on 12.0-CURRENT on the drm-next-4.7 branch, built on 28-October)
mounting the home directories over NFSv4.  At first, everything is
fine and performance is good.  But if the client does a buildworld
using sources on NFS and locally stored objects, performance slowly
degrades.  The degradation is most noticeable with metadata-heavy
operations.  For example, "ls -l" in a directory with 153 files takes
less than 0.1 seconds right after booting.  But the longer the
buildworld goes on, the slower it gets.  Eventually that same "ls -l"
takes 19 seconds.  When the home directories are mounted over NFSv3
instead, I see no degradation.

top shows negligible CPU consumption on the server, and very high
consumption on the client when using NFSv4 (nearly 100%).  The
NFS-using process is spending almost all of its time in system mode,
and dtrace shows that almost all of its time is spent in
ncl_getpages().

I have delegations disabled on the server.  On the client, the home
directories are nullfs mounted to two additional locations, and the
buildworld was actually using one of those nullfs mounts, not the NFS
mount directly.

Any ideas?

-Alan