An interesting question came up in the Google Webmaster Office hours – if a site has a slow loading resource, such as javascript, that does not break the render, is it okay to block Googlebot from crawling it because it is slow and could impact the page negatively with regards to page speed?

First, it is worth noting that just because a blocked javascript (or other resource) doesn’t affect the rendering, because Googlebot cannot see that javascript, it has no way of knowing what it does or how it may impact the page.  And Mueller advises that it can certainly affect whether Google sees a site as mobile friendly or not.

I’d be careful with blocking scripts just for Google because then it might happen that we can’t render the page properly, that we can’t see like the full layout properly and the tricky part there is if we can’t see the full layout properly, we don’t know if the page is mobile friendly or not.  So especially on mobile, you wouldn’t have that kind of mobile boost because we don’t know is it mobile friendly or is it not mobile friendly.  We can’t see the page properly.

So I’d avoid doing that there.

He also talks about the mobile speed specifically.  And he mentions that because Google also uses data from Chrome with regards to speed, it would be harder to “cheat”, so to speak, by blocking scripts in an attempt to make Google think a page loads faster than it really does.

Also, with regards to speed, at the moment, we differentiate between kind of reasonably okay fast and really, really slow.  So something that takes like multiple minutes to actually load a page, that’s something we would say it’s probably pretty slow compared to the rest of the sites.

With the change to mobile speed, we’ll look at that a little bit more differentiated, but we use a number of factors with regards to speed there, including some factors that we see from Chrome, where we see what users are actually seeing.

So just by blocking specific scripts to try and make the page look faster for Googlebot probably won’t necessarily change anything with regards to how we see the speed for your site.  Instead, I’d try to see if there are ways to speed up the site in general for both users and for Google.

So while it might be tempting to block slow resources you think Googlebot will score negatively, it would be worth to spend the time speeding the resource up, rather than blocking it and running the risk of losing the mobile friendly ranking boost.