From semantic-mediawiki.org

Configuration parameter details:
Name $smwgRSSEnabled
Description Sets whether RSS is enabled or not
Default setting
Software Semantic MediaWiki
Since version
Until version
Configuration RSS export · inline queries
Keyword auto refresh

RSS as such does not cause more load than special page "Ask"Provides an interface that assists users with creating and executing semantic queries but it might attract more users. The configuration parameter was introduced in Semantic MediaWiki 1.0Released on 31 December 2007 and compatible with MW 1.9.x - 1.12.x. and removed in Semantic MediaWiki 1.8.0Released on 2 December 2012 and compatible with MW 1.17.0 - 1.22.x
(except 1.22.1).
meaning that RSS will always be enabled.