Lin545: For whatever reason, this seems to help in very dense cities.

Go to Project Zomboid install directory, locate "ProjectZomboid64.json" and change memory value from default 1024M to higher setting, like 4096M for 4GB or 8192M for 8GB process allocation.

Note that this may interfere with MP as servers may CRC-check your files to match.

So backup maybe.

Kaguya-hime: Which string is responsible for a memory allocation?
https://kek.gg/i/JRgNg.png

Lin545: New version seem to have changed this file:
https://github.com/GameServerManagers/LinuxGSM/issues/1626

Please try this instead: https://pzwiki.net/wiki/Startup_Parameters
You need to edit .desktop file/shortcut for this and add for example this parameters: -Xmx4096m -Xms4096m --
See the last link, paragraph "JVM Arguments" for details.