Win11 start menu is a react native component.

Users report that clicking the Start button can spike CPU usage by 30% to 70% on at least one core, depending on the hardware configuration. The issue doesn't occur consistently across all systems, with some users noting it happens in about 50% of clicks.

Quotes from: https://winaero.com/windows-11-start-menu-revealed-as-resource-heavy-react-native-app-sparks-performance-concerns/