Vercel ISR page isn't pre-rendered

I expect an ISR page to be pre-rendered at build time, so that it loads instantly on the first visit.

But it's not the case, the first load is slow. To reproduce the issue:

First redeploy the app to reset the ISR cache : (wait ~20s for Vercel deployment to complete)
Then visit the ISR page. It takes some time to load the first time.
Subsequent visits are fast as the ISR page is cached.