The indexing is handled by apache solr and java - thus whatever resources that would consume is what you need to research when it comes to affects upon performance of the site.
If running linux and one has access to command line, run top from an ssh session. Then in another ssh session, execute the task associated with global search. You should see java kick in.
So if your server is an all in one .... ie, web (moodle code), DB server on same box, obviously running another daemon will eat up more memory and when it executes more cpu cycles.
Been my experience (on 16Gig memory boxen) that indexing of new content is usually pretty fast and while there is a spike it's not been something that one notices overall. Obviously, an all in one server running 4 or 8 Gig may not perform as well.
'spirit of sharing', Ken