odd NFS v4.2 regression 4.4..5.15

Greetings network wizards,

I seem to have stumbled across a kind of odd looking NFS regression
while looking at something unrelated.

I have a few VMs that are mirrors of my desktop box, including some
data, but they usually over-mount directories with tons of data via
NFS, thus hang off the host like a mini-me Siamese twin for testing.

In the data below, I run a script in git-daemon's home to extract RT
patch sets with git-format-patch, on the host for reference, on the
host in an NFS mount of itself, and the mini-me VM, where I started.

Note: bonnie throughput looked fine for v4.2.

host
time su - git -c ./format-rt-patches.sh
real    1m5.923s
user    0m54.692s
sys     0m11.550s

NFS mount my own box, and do the same from the same spot
time su git -c 'cd /homer/home/git;./format-rt-patches.sh'

4.4.231     v4.2        v3
1   real    2m27.046s   2m2.059s
    user    0m59.190s   0m58.701s
    sys     0m41.448s   0m32.541s

5.15-rc5    v4.2        v3
   real     3m14.954s   2m8.366s
   user     0m59.901s   0m58.317s
   sys      0m52.708s   0m32.313s
        vs 1   0.754       0.951

repeats     v4.2        v3
   real     3m16.313s   2m7.940s
   real     3m10.905s   2m8.029s

----

guest
time ./format-rt-patches.sh

host=4.4, mounted v4.2
          guest=5.15  guest=4.4  guest vs guest
1 real    3m38.334s   3m11.383s  .876
  user    1m7.747s    1m2.666s
  sys     0m53.721s   0m48.127s

host=4.4, mounted v3
          guest=5.15  guest=4.4
2 real    2m58.458s   2m46.857s  .934
  user    1m6.660s    1m3.685s
  sys     0m43.026s   0m42.657s
      vs 1   1.223       1.146

host=5.15,mounted v4.2
          guest=5.15  guest=4.4
3 real    4m49.320s   4m29.317s  .930
  user    1m5.740s    1m4.724s
  sys     1m12.240s   1m6.544s hmm..
      vs 1   0.754      0.710

  real    4m48.886s
  user    1m6.503s
  sys     1m14.910s *

  real    4m46.432s
  user    1m5.877s
  sys     1m15.251s * odd, only in guest

host=5.15,mounted v3
          guest=5.15  guest=4.4
4 real    3m6.965s    2m56.527s  .944
  user    1m5.192s    1m3.353s
  sys     0m45.775s   0m43.361s
      vs 2   0.954       0.945