Network performance issue with 2nd server

ODA X-4 running 12.1.2.2.0.

Weird little problem. ssh connection (from any client) to the oak1 server is immediate. ssh connection to oak2 is quite slow (several seconds). ssh uses trusted keys (identically configured on both).

Running ssh with debug switch -vv shows the delay occurs at the following step:

 debug1: Entering interactive session.

On oak1 this is immediate followed by the next debug messages:

debug2: callback start
debug2: fd 3 setting TCP_NODELAY

On oak2 there is a noticeable delay of a few seconds, before displaying the next 2 debug messages.

This is a fairly new issue - last week and prior, ssh connectivity to both servers were normal and without delays. Uptime is identical.