If you take a look at their extension, it injects a number of separate content scripts on the document_start event for a webpage. This blocks document loading and Google's recommendation is to load content scripts on document_idle which would not block loading.

These scripts that are injected into every page (including all iframes) include this one which is a 25,000 line file:

https://crxcavator.io/source/hdokiejnpimakedhajhdlcegeplioah...

Things like this can have large impact on performance. My company develops a Chrome extension and we are very focused on optimizing the performance of our content script and minimizing its impact on host pages. We handwrite all the Javascript code in the content script and manually include the very few third party dependencies we have in it. Yes, it would be a lot easier to use NPM with Webpack to build the content script file, but it's important to keep content scripts as slim as possible.