NFS Client – Intermittent Performance Issues

I have a weird issue with one of the NFS clients (Ubuntu 16.04 LTS).
I have been working for the past few days to try and debug this, but so far without success.
After I mount the partition, everything works perfectly for a few days, transfer speed is 1 Gbps between the client and server. After a few days the speed drops to less than 10 mbps and even a simple directory listing takes a few seconds and I/O wait is at 100%

What I noticed is that the backlog wait, especially for the write operations is extremely high:

root@srv:~# mountstats /mnt/data
Stats for 192.168.0.15:/mnt/data mounted on /mnt/data:
  NFS mount options: rw,vers=4.0,rsize=16384,wsize=16384,namlen=255,acregmin=3,acregmax=60,acdirmin=30,acdirmax=60,hard,proto=tcp,port=0,timeo=600,retrans=2,sec=sys,clientaddr=192.168.0.150,local_lock=none
  NFS server capabilities: caps=0xffdf,wtmult=512,dtsize=16384,bsize=0,namlen=255
  NFSv4 capability flags: bm0=0xfdffbfff,bm1=0xf9be3e,bm2=0x0,acl=0x3,pnfs=notconfigured
  NFS security flavor: 1  pseudoflavor: 0

NFS byte counts:
  applications read 8168679407142 bytes via read(2)
  applications wrote 4833000353435 bytes via write(2)
  applications read 0 bytes via O_DIRECT read(2)
  applications wrote 0 bytes via O_DIRECT write(2)
  client read 4218977852758 bytes via NFS READ
  client wrote 4832098253207 bytes via NFS WRITE

RPC statistics:
  561421762 RPC requests sent, 561421608 RPC replies received (1 XIDs not found)
  average backlog queue length: 0

READ:
        263822474 ops (46%)     0 retrans (0%)  0 major timeouts
        avg bytes sent per op: 184      avg bytes received per op: 16051
        backlog wait: 8.772689  RTT: 27.972131  total execute time: 36.752241 (milliseconds)
WRITE:
        295296111 ops (52%)     0 retrans (0%)  0 major timeouts
        avg bytes sent per op: 16567    avg bytes received per op: 132
        backlog wait: 62468603019.791718        RTT: 78.030143  total execute time: 62468603097.830574 (milliseconds)

There are no errors, no warnings, I tried debugging with "echo 1 > /proc/sys/vm/block_dump" (which has done miracles for me in the past), but this time there's nothing visible related to NFS.
Any idea how I can debug further and see what's causing the extremely high backlog wait?