It slows the browser A LOT when trying to find anything on a large web page (the most popular use case). It is probably because of "search as you type" functionality. It should be possible to deactivate it. The other possibility is to perform "search as you type" only for expressions longer than e.g. 3 characters. I dislike also a very broad permissions this addon requires. It can be a security concern. Addon named "Multiple Highlight" shows that it is possible to search RegEx without any permission requested.