We'll create fresh WordPress site with Disable Lazy Loading for IFRAMES installed. You have 20 minutes to test the plugin after that site we'll be deleted.
After WordPress 5.7 is released, iframes will have the loading="lazy"
attribute added by default. See Trac ticket 50756, and changeset 49808 for the actual implementations in core.
This plugin, for the short term, is a global disabling of lazy loading for all iframes, no matter their context.
Changelog