large sitemap index woes

hey folks,

we've made some progress on the dynamic sitemap generation, but now we're hitting a wall with the sitemap index for really large sites. it's causing some serious headaches.

• The Core Problem: our main sitemap index file is either taking forever to build/update, or it's not correctly reflecting the sub-sitemaps changes fast enough.
• Symptoms & Technical Details:
  • Slow generation times, sometimes leading to server timeouts.
  • Google Search Console reports 'couldn't fetch' for the sitemap index occasionally.
  • We're seeing stale lastmod dates for some of the sub-sitemaps in the index, even after they've been updated. its really frustrating.
• Example (Illustrative):

  <?xml version="1.0" encoding="UTF-8"?>
  <sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
    <sitemap>
      <loc>https://example.com/sitemap_products_1.xml.gz</loc>
      <lastmod>2023-10-26T10:00:00+00:00</lastmod> <!-- This date is often stale -->
    </sitemap>
    <sitemap>
      <loc>https://example.com/sitemap_products_2.xml.gz</loc>
      <lastmod>2023-10-26T10:00:00+00:00</lastmod>
    </sitemap>
    <!-- ... hundreds more ... -->
  </sitemapindex>
  

  and sometimes, we get a timeout:

  [2023-11-01 04:30:15] cron.ERROR: SitemapIndexGenerationFailed: Process timed out after 300 seconds.
  

• Our Current Approach: we're using a database query to build the sitemap index, aggregating lastmod from individual sitemap files.
• Seeking Advice On:
  • Strategies for optimizing sitemap index generation speed for millions of URLs.
  • Best practices for ensuring lastmod dates in the sitemap index are always accurate and timely.
  • Any caching mechanisms or distributed generation techniques for massive sitemap index files.
• Closing: Really looking for some expert insights here.