[CentOS] Slow performance with NFSv4.1 on CentOS 7.5 ?

We have a number of identical NFS clients mounting a server using 
NFSv4.1 - server and clients are all running CentOS 7.5 (kernel 
3.10.0-862.14.4.el7.x86_64)

However, on some clients, the NFS performance 'degrades' with time ...

Running a simple test - a python script that just imports a module 
(python and its modules are installed on the NFS share) can be an order 
of magnitude or more slower on some clients. i.e. very little data is 
transferred, it is the rate of stat'ing and opening files on the NFS 
server that is 'slow'

Running a tcpdump on a 'slow' client shows that the NFS traffic 
generated on the 'slow' client is again an order of magnitude or more 
when compared with that generated by a 'fast' client

The majority of the extra NFS traffic in the slow case, appears to be a 
large number of NFS 'TEST_STATEID' calls the client makes - which are 
not there in the tcpdump on the fast client

The issue can be 'fixed' in the short term by rebooting the affected 
client - and after a reboot, running the same tcpdump shows no 
TEST_STATEID calls - however after a while (several days), the 
performance might degrade again

I've found a number of reports of excessive TEST_STATEID calls - but 
most seem to relate to NFSv4 client hangs - which is not happening here 
- things are working, but much slower than they should be ...

Has anyone come across this issue - and have any fixes/workarounds?

Thanks

James Pearson