NFS poor write performance

You use the sync-option in your export statement. This means that the server only confirms write operations after they are actually written to the disk. Given you have a spinning disk (i.e. no SSD), this requires on average at least 1/2 revolution of the disk per write operation, which is the cause of the slowdown.

Using the async setting, the server immediately acknowledges the write-operation to the client when it is processed but not yet written to the disk. This is a little bit more unreliable, e.g., in case of a power failure when the client received an ack for an operation that did not happened. However, it delivers a huge increase in write-performance.

(edit) I just saw that you already tested the options async vs sync. However, I am almost sure that this is the cause of your performance degradation issue -- I once had exactly the same indication with an idencitcal setup. Maybe you test it again. Did you give the async option at the export statement of the server AND in the mount operation at the client at the same time?

It can be a problem related to packet size and latency. Try the following:

• enable jumbo frames (MTU >= 9000 bytes) on both machines
• use UDP or, alternatively, manually increase TCP window size on both machines

The report back your results.