Out of memory errors in XML sitemaps after upgrading from Yoast SEO 6 to 9.2.1

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Please give us a description of what happened.

After upgrading from WP SEO 6, which I've hung on to for a while because of several performance issues that weren't fixed until v9.1 or v9.2, https://www.androidpolice.com/sitemap_index.xml started returning an error 500. The details are:

Allowed memory size of 134217728 bytes exhausted (tried to allocate 20480 bytes)

You can see the Sentries for it here:
https://sentry.io/share/issue/e48ab4a472d743bca4af14bc7d7d0310/
https://sentry.io/share/issue/254e77128fbb4fd49d162e5a2d32d459/

This was never a problem before.

Note: I do use W3 Total Cache in case it matters.

I would maybe expect an OOM to happen in individual sitemap shards, like https://www.androidpolice.com/post-sitemap1.xml, which is why the setting for the number of posts per sitemap used to exist (and I had to bump it down to 500), but those actually seem to load OK. It's just the master sitemap index that isn't loading.

Technical info

Used versions

• WordPress version: 4.9.8
• Yoast SEO version: 9.2.1
• Relevant plugins in case of a bug:

W3 Total Cache 0.9.6.1

• Tested with theme: custom

Start by reproducing the 500 and memory exhaustion at androidpolice.com/sitemap_index.xml, then compare it with the working post-sitemap1.xml shard under the reported WordPress 4.9.8 and Yoast SEO 9.2.1 setup. Done means the master sitemap index loads without exhausting memory, including when W3 Total Cache is enabled.