Those six hours is the reason why temporary file aren't immediately deleted when cron tasks run. If then there are many temporary file to delete, a single run isn't sufficient to delete all the files.
Start with clean URLs, add Pathauto and Metatag modules, submit your XML sitemap to Google Search Console, and keep your content original and well-structured.
Hi. I use the "[node:title] | [site:name]" tokens for the main image of news articles, blog posts, and similar publications. It's a quick and practical automatic ALT.
https://backdropcms.org/project/metatag
https://backdropcms.org/project/xmlsitemap
https://backdropcms.org/project/imagefield_tokens
These are more powerful modules, in my opinion...
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The cron job system_cron() will delete temporary files after the max time stipulated by a constant (6 hours)
Those six hours is the reason why temporary file aren't immediately deleted when cron tasks run. If then there are many temporary file to delete, a single run isn't sufficient to delete all the files.