Google has a legacy Search Console feature to let you limit, or slow down, how fast Google can crawl your site under the crawl rate setting. This feature, as we know, takes a day or so to kick in, and does not speed up crawling, it only can slow crawling of your site.
You can access the setting over here and this is what it looks like:
As you can see, it says “limit Google’s maximum crawl rate” not speed up the crawl rate.
John Mueller said all it does is limit the QPS, does that mean crawling per second? I think normally QPS means queries per second, but we get it.
It is best if you do not touch this setting, if you can avoid it. Google’s help document says “if your crawl rate is described as “calculated as optimal,” the only way to reduce the crawl rate is by filing a special request. You cannot increase the crawl rate. Otherwise, select the option you want and then limit the crawl rate as desired. The new crawl rate will be valid for 90 days.”
Forum discussion at Twitter.
