Continuing on from my previous unanswered post:
Did some more testing, this time using two machines on Apache 2.0.55 and PHP 4.4.2. One however running MYSQL 3.23.58 and the other running MYSQL 4.1.16.
The one running the newer version of MYSQL takes a long time for a comment to be published. Everything else works fine.
So in conclusion, I have found out the following re: performance of comments -
Apache 2.0.55 + MYSQL 4.x --> slow
Apache 2.0.55 + MYSQL 3.23.* --> normal
Apache 1.3.34 + MYSQL 4.x --> normal
Haven't test out Apache 1.3.34 with MYSQL 3.23.x though. SSL or no SSL does not make a difference as thought so.
I run a variety of web apps on the above 3 machines and they are all ok with no noticable performance difference.
This is not a show stopper but it would be good to have some idea why this is happening with WordPress. Thanks!