I installed cachefilesd on an NFS client using standard fstab options:
fsc,rw,defaults,_netdev.

Client and server are connected on a local network using a Gigabit
connection. Files transfer without cachefilesd are quick, but opening a
file is very slow for big files when cachefilesd is activated: 10 seconds
for a 4 MBytes file, 60 seconds for a 50 MBytes file. No error is displayed.

After investigating, it seems that:
 - transfered files are copied into cachefilesd local cache (in
/var/cache/fscache)
 - opening a big file is always very long, even when the file is in the
local cache.

Client reboot doesn't changes anything.

FSCache stats are as follow, but I don't know how to analyze them:

  FS-Cache statistics
  Cookies: n=163 v=1 vcol=0 voom=0
  Acquire: n=971 ok=971 oom=0
  LRU    : n=0 exp=2128 rmv=96 drp=148 at=0
  Invals : n=8852
  Updates: n=3481 rsz=0 rsn=0
  Relinqs: n=808 rtr=0 drop=808
  NoSpace: nwr=0 ncr=0 cull=2
  IO     : rd=1696879 wr=1696352
  RdHelp : RA=0 RP=0 WB=0 WBZ=0 rr=0 sr=0
  RdHelp : ZR=0 sh=0 sk=0
  RdHelp : DL=0 ds=0 df=0 di=0
  RdHelp : RD=0 rs=0 rf=0
  RdHelp : WR=0 ws=0 wf=0

The same configuration was working perfectly with a previous Debian release
(Stretch 9.13).