Increased memory usage during install

In Drupal 8.9.x we are seeing a tenfold increase in memory consumption when installing a project using existing config. Using git bisect it appears this problem has been introduced in #474684: Allow themes to declare dependencies on modules.

Please note that I have been testing a project that uses a composer based workflow and the listed commits are from the drupal/core subtree split.

Last known good commit

• Commit: 353773bfbc494c6d1cc4fd33c4463e3307434a24
• Issue: SA-CORE-2020-001
• Memory usage: 150113928 (142.2MB)
• Peak memory usage: 175713912 (167.6MB)

    [drush] Executing command: /home/pieter/v/joinup-dev/vendor/bin/drush --verbose --yes --root="/home/pieter/v/joinup-dev/web" --db-url="mysql://root:@127.0.0.1:3306/joinup" --site-name="Joinup" --account-name="admin" --account-pass="admin" --account-mail="admin@example.com" --existing-config site-install joinup install_configure_form.enable_update_status_module=NULL install_configure_form.enable_update_status_emails='NULL' install_settings_form.sparql.host=localhost install_settings_form.sparql.port=8890 install_settings_form.sparql.namespace=Drupal\\Driver\\Database\\joinup_sparql
 [info] Executing: command -v mysql
 [info] Executing: mysql --defaults-file=/tmp/drush_Q3FA1B --database=joinup --host=127.0.0.1 --port=3306 --silent -A < /tmp/drush_v52JDB

 // You are about to DROP all tables in your 'joinup' database. Do you want to continue?: yes.

 [info] Sites directory sites/default already exists - proceeding.
 [info] Executing: mysql --defaults-file=/tmp/drush_A5ztzC --database=joinup --host=127.0.0.1 --port=3306 --silent -A < /tmp/drush_ZS6uGF
 [info] Executing: mysql --defaults-file=/tmp/drush_D6YPWC --database=joinup --host=127.0.0.1 --port=3306 --silent -A < /tmp/drush_MDWvmC
 [info] Executing: mysql --defaults-file=/tmp/drush_KyY0aF --database=joinup --host=127.0.0.1 --port=3306 --silent -A < /tmp/drush_izwmfC
 [info] Installing from existing config at /home/pieter/v/joinup-dev/config/sync
 [notice] Starting Drupal installation. This takes a while.
 [success] Installation complete.
 [success] Memory usage: 150113928, peak memory usage: 175713912

First bad commit

• Commit: 6e9d3dea72537b3a557749c5e614abc92f55b8bf
• Issue: #474684: Allow themes to declare dependencies on modules
• Memory usage: 1378101456 (1314.3MB)
• Peak memory usage: 1403767040 (1338.7MB)

    [drush] Executing command: /home/pieter/v/joinup-dev/vendor/bin/drush --verbose --yes --root="/home/pieter/v/joinup-dev/web" --db-url="mysql://root:@127.0.0.1:3306/joinup" --site-name="Joinup" --account-name="admin" --account-pass="admin" --account-mail="admin@example.com" --existing-config site-install joinup install_configure_form.enable_update_status_module=NULL install_configure_form.enable_update_status_emails='NULL' install_settings_form.sparql.host=localhost install_settings_form.sparql.port=8890 install_settings_form.sparql.namespace=Drupal\\Driver\\Database\\joinup_sparql
 [info] Executing: command -v mysql
 [info] Executing: mysql --defaults-file=/tmp/drush_FQExgb --database=joinup --host=127.0.0.1 --port=3306 --silent -A < /tmp/drush_a2dbk9

 // You are about to DROP all tables in your 'joinup' database. Do you want to continue?: yes.

 [info] Sites directory sites/default already exists - proceeding.
 [info] Executing: mysql --defaults-file=/tmp/drush_d4t6b7 --database=joinup --host=127.0.0.1 --port=3306 --silent -A < /tmp/drush_xxtsx9
 [info] Executing: mysql --defaults-file=/tmp/drush_raMCa9 --database=joinup --host=127.0.0.1 --port=3306 --silent -A < /tmp/drush_NoSTL6
 [info] Executing: mysql --defaults-file=/tmp/drush_cL66g7 --database=joinup --host=127.0.0.1 --port=3306 --silent -A < /tmp/drush_ZZdGsa
 [info] Installing from existing config at /home/pieter/v/joinup-dev/config/sync
 [notice] Starting Drupal installation. This takes a while.
 [success] Installation complete.
 [success] Memory usage: 1378101456, peak memory usage: 1403767040

The project that is affected by this problem is open source, ref: https://github.com/ec-europa/joinup-dev. Ticket tracking this problem is https://github.com/ec-europa/joinup-dev/pull/2088.