Out of memory

Problem/Motivation

On a site we have ~ 20k taxonomy terms in our "tag" taxonomy. The term merge functionality totally breaks if there is too many taxonomies.
PHP message: PHP Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 950272 bytes) in /var/www/html/web/core/lib/Drupal/Core/Render/Renderer.php on line 493

Steps to reproduce

Have ~20K taxonomies, then go to /admin/structure/taxonomy/manage/{taxonomy}/merge

Proposed resolution

I propose not creating 20k checkboxes if it causes out of memory issues. Maybe using entity_reference_autocomplete instead?

Remaining tasks

User interface changes

API changes

Data model changes