Hi ,
If we take a simultaneous packet trace from NetApp and RHEL client reproducing the good and bad scenario, we can find what is the reason for the delay seen with different client versions.
Since ONTAP code is same but with different client versions we notice different delays, this looks to be some change in the client which could be causing this.
I would suggest to open a ticket so that support can look at the packet traces , compare and provide an analysis.