Most people don't know this but Chrome supports GM scripts natively - just save the file as `whatever.user.js` and drag + drop it onto the extensions window.
One downside of this approach: it redirects to the mobile interface after the HTTP request and DOMReady
This extension redirects requests before they've even started, making it much faster than what's possible with a Greasemonkey script. Greasemonkey scripts are also hard to discover and install these days.
Userscripts can also run before any content scripts through `@run-at document-start`. With a browser extension, you're giving up portability, security (an extension can wreck a lot more havoc than a site-specific script) and the freedom to publish without being dependant on a "store".
This extension redirects before a request is even made. `document-start` runs when the whole page is downloaded, but before the DOM is constructed, which is a lot slower. As for portability, both Firefox and Edge are committed to supporting Web Extensions, which is pretty much the Chrome extension API. This extension is not tied to any store. You can install it manually exactly as you would with a userscript. Actually, userscripts in Chrome, are just re-packaged as native extensions on install.
To your question: A lot. How easy is it to install a chrome plugin vs. install a packaged, auto-updating Greasemonkey script? What is the real difference in performance?