Relevanssi now has this functionality built in. Please upgrade your Relevanssi and remove this one.
Relevanssi has the ability to build the index in a step wise fashion (by default 500 posts at a time). However if you have a larger number of posts you can be stuck clicking the “Continue Indexing” button over and over again.
Step Index adds a small peice of JavaScript that will do this process automatically for you if you check the “Step Index” checkbox before clicking “Continue Indexing”.