Can low NFS IO speeds effect client host performance when commands like tar is running?

We have NFS used over firewall which is limiting the performance.

And we have this occasional scenarios of load increasing on the client host, whenever some huge IO operation is being done like tar.

My understanding is that tar can cause congestion and thus affecting other NFS operations.

And as the home directory of users is also on NFS and with the new poor performance of NFS (caused due to congestion with tar command) causes normal operations like ssh, su, ls etc. also to be slow and as in prouduction environment, these operations could be high and subsequently more operations are waiting to be done at the same time, increase load average. This increase in load average is found in sar reports.

But what I am not clear is where is the congestion actually created by tar? Is it inside the NFS storage (netapp in our case) or in the network?

My above hypothesis is correct only if the congestion happens on network, as we don't see any performance effect on other NFS clients at the time (as if the congestion is present in the storage server, all clients should be effected).

Also, I am not sure how to check if there is network congestion between the client and server if my hypothesis is right.