To add a bit of debugging I did (using two VMs):
- There server side kernel version does not matter (tested trusty and utopic)
- A client  with a 3.13 kernel will have the delay, while the 3.16 kernel is ok

Enabling NFS debugging on both clients (as root "echo 32767
>/proc/sys/sunrpc/nfs_debug") shows an interesting fact:

3.13 client:
 7581.776148] NFS: nfs4_discover_server_trunking: testing 'lam-utopic6401'
 7581.776154] NFS call  setclientid auth=RPCSEC_GSS, 'Linux NFSv4.0
 7596.776086] RPC: AUTH_GSS upcall timed out.
 7596.776086] Please check user daemon is running.
 7596.776110] NFS reply setclientid: -13
 7596.776121] NFS call  setclientid auth=RPCSEC_GSS, 'Linux NFSv4.0
 7597.024109] NFS reply setclientid: -13
 7597.024766] NFS call  setclientid auth=UNIX, 'Linux NFSv4.0
 7597.025506] NFS reply setclientid: 0
 7597.025512] NFS call  setclientid_confirm auth=UNIX, (client ID fb69c653040000
 7597.026068] NFS reply setclientid_confirm: 0

3.16 client:
 2137.866775] NFS: nfs4_discover_server_trunking: testing 'lam-utopic6401'
 2137.866783] NFS call  setclientid auth=UNIX, 'Linux NFSv4.0
 2137.867420] NFS reply setclientid: 0
 2137.867426] NFS call  setclientid_confirm auth=UNIX, (client ID fb69c653050000
 2137.867727] NFS reply setclientid_confirm: 0

So the newer kernel seems to skip the other authentication methods. And
of those two AUTH_GSS is causing the delay because it times out.
Unfortunately I did not spot any commits in between 3.13 and now that
would immediately sound like a good candidate for fixing this.

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  Long delay when mounting NFS shares

Status in “linux” package in Ubuntu:

Bug description:

  After upgrading the kernel on Ubuntu 14.04 to 3.13.0-29-generic I
  found that it can take a considerably longer amount of time to mount
  NFS shares.

  How to reproduce:

  1. Install Ubuntu 14.04 on two machines. A minimal server install should be 
fine. One will act as a server, the other as a client.
  2. Upgrade the kernel on the client machine to at least 3.13.0-27-generic.
  3. Install the nfs-kernel-server package on the server.
  4. Add a directory to /etc/exports and run service nfs-kernel-server start.
  5. Install the nfs-common package on the client.
  6. Attempt to mount the shared directory on the client.

  Expected result:

  The shared directory should be mounted on the client more or less

  Actual result:

  The mount command hangs and eventually completes. In this test case it
  takes 16 seconds consistently. On a production machine where the
  problem was first discovered the time is considerably longer and
  essentially makes it impossible to mount NFS shares.


  I have been able to reproduce the problem on clients running Linux
  3.13.0-27-generic and later. The problem is not reproducible on
  clients running Linux 3.13.0-24-generic. The problem is reproducible
  no matter which kernel version is used on the server.

  Tested client kernel versions:

  Linux 3.13.0-29-generic #53-Ubuntu  FAIL
  Linux 3.13.0-27-generic #50-Ubuntu  FAIL
  Linux 3.13.0-24-generic #47-Ubuntu  OK

  Tested server kernel versions:

  Linux 3.13.0-29-generic #53-Ubuntu  OK
  Linux 3.13.0-27-generic #50-Ubuntu  OK
  Linux 3.13.0-24-generic #47-Ubuntu  OK

  lsb_release -rd:

  Description:  Ubuntu 14.04 LTS
  Release:      14.04

  apt-cache policy linux-image-generic:

    Installed: (none)
    Version table: 0
          500 trusty-updates/main amd64 
          500 trusty-security/main amd64 
Packages 0
          500 trusty/main amd64 Packages

  ProblemType: Bug
  DistroRelease: Ubuntu 14.04
  Package: linux-image-3.13.0-29-generic 3.13.0-29.53
  ProcVersionSignature: Ubuntu 3.13.0-29.53-generic
  Uname: Linux 3.13.0-29-generic x86_64
   total 0
   crw-rw---- 1 root audio 116,  1 Jun  7 16:26 seq
   crw-rw---- 1 root audio 116, 33 Jun  7 16:26 timer
  AplayDevices: Error: [Errno 2] No such file or directory: 'aplay'
  ApportVersion: 2.14.1-0ubuntu3.2
  Architecture: amd64
  ArecordDevices: Error: [Errno 2] No such file or directory: 'arecord'
  AudioDevicesInUse: Error: [Errno 2] No such file or directory: 'fuser'
  CRDA: Error: [Errno 2] No such file or directory: 'iw'
  Date: Sat Jun  7 16:30:02 2014
  HibernationDevice: RESUME=UUID=9c13108b-d40a-4881-b563-c477ed2e6804
  InstallationDate: Installed on 2014-06-07 (0 days ago)
  InstallationMedia: Ubuntu-Server 14.04 LTS "Trusty Tahr" - Release amd64 
  IwConfig: Error: [Errno 2] No such file or directory: 'iwconfig'
   Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
   Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
   Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
   Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
  MachineType: QEMU Standard PC (i440FX + PIIX, 1996)
  ProcFB: 0 VESA VGA
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.13.0-29-generic 
root=UUID=9068010d-8332-47c8-ae62-ccc62ca57290 ro
   linux-restricted-modules-3.13.0-29-generic N/A
   linux-backports-modules-3.13.0-29-generic  N/A
   linux-firmware                             N/A
  RfKill: Error: [Errno 2] No such file or directory: 'rfkill'
  SourcePackage: linux
  UpgradeStatus: No upgrade log present (probably fresh install) 01/01/2011
  dmi.bios.vendor: Bochs
  dmi.bios.version: Bochs
  dmi.chassis.type: 1
  dmi.chassis.vendor: Bochs
dmi:bvnBochs:bvrBochs:bd01/01/2011:svnQEMU:pnStandardPC(i440FX+PIIX,1996):pvrpc-i440fx-2.0:cvnBochs:ct1:cvr: Standard PC (i440FX + PIIX, 1996)
  dmi.product.version: pc-i440fx-2.0
  dmi.sys.vendor: QEMU

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