I didn't say that it has now effect, i only said that the effect is not as big as it using a seperate table for the archiv.
Your method improves the speed a bit, cause you might avoid a full table scan. That depends on the query and the indices you have set.
A proper mysql setup and configuration is important also, but i can't say how this effects your results without knowing your settings and machine.
My board has nearly one million posts in the database. Restricting the displayed post per forum affects speed. But to get a significant improvment, i'll have to remove the old posts from the posts table.
Indexing has a positve effect on similar queries, but a large table produces more load as a smaller one. Especially on searches, which are the bottleneck of threads. Submitting a several searches in a short amount of time can hook up every db server with a large posting table.
