Regarding crawling spikes being unrelated to Google's broad core updates, it's also important to understand that recent changes on a site would *not* typically be reflected in major algorithm updates (like broad core updates). Here's about that:
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Via @johnmu: Major impact from an algo update wouldn't be from *recent* changes. For larger sites, it can take Google's algorithms a longer time to adjust to site changes. It could take several months to recrawl, reindex, & reprocess the site changes: glennga.be/2FTWz71
GIF
