I think it would nice to create a static rss page like rss.xml (updated only when new posts are made) instead of a php script, as it would reduce server load to a great extent.

You can create such page yourself with the RSS plugin - in "Downloads". It's exactly for this purpose :)

The authors of miniBB did the same on the front page - top right.

I know and I have implemented it on my site. However that's a php script right? I mean it has to query the database everytime the file is requested. What I would like to see is a static rss page, like a html page. The rss page would be updated ONLY when the database gets updated, hence no need to query again and again.

It should be updated when database is updated. But also it needs to be updated when some topic is deleted, or edited, or moved to the section. And if we have many forums in RSS feed? And what about users who have PHP in safe mode, which resctricts to create files?..

And what about, if we store HTML cache for each forums page? It is much faster. But is it simplier to implement?