When combine: true is not defined in any extensions yui.yml file it becomes false and means UI will experience slower performance.

Steps to reproduce:
1. Create a PlatformUI extension bundle with an yui.yml file config, that adds new modules or extending existing ones,
2. Clear cache and regenerate the assets,
3. When loading PlatformUI app notice the app performance decrease (files from extensions are not combined but loaded individually, sometimes even from CDN as reported in EZP-26768)

Instead of creating new extensions you might want to include eZ Studio bundle or Content on the Fly bundle for the reproduction.