Bug: Async + View Transitions slows down page navigation

[luppiale]

Describe the bug

If you configure your external layout to enable the view transition api (doc) and you have the async experimental flag turned on, the navigation between pages becomes really slow even if you don't have any data loading.

Reproduction

stackblitz

Just comment out the document.startViewTransition call in the external layout or turn off the experimental async flag to have the navigation working as expected

System Info

It "works" on firefox because it doesn't support view transitions

Severity

blocking an upgrade
blocking the usage of async features

[gterras]

An experimental flag cannot "block an update" btw.

[paoloricciuti]

Screen.Recording.2025-07-18.at.14.04.23.mov

Curiously I can reproduce in stackblitz but not locally

[gornostay25]

any updates?

[paoloricciuti]

Ugh didn't have time to check... I'll try to find some time tomorrow

[sillvva]

Ugh didn't have time to check... I'll try to find some time tomorrow

Here's a more update-to-date MRE. Issue still exists.