As far as I know, macros are run when a tiddler using that macro is added to the story river, and thus wikified, or otherwise accessed through transclusion from another tiddler opened in the story river. Or if it is being accessed from a template that makes up part of your UI. I believe the running time you are seeing in the performance plugin is referring to the accumulated time that that macro was run each time a tiddler is opened that uses that macro, and not continuous running time in the background. A macro is not an executable program like a service or daemon that runs in the background waiting to act on a tiddler.
You might want to search your wiki and find any tiddler that is calling that macro and make sure there is not some template that is always being displayed in your UI that is constantly calling that macro over and over and over for some reason, which would give the impression that it is constantly running in the background.