Safari 26.0.1: __prerender_bypass cookie present in production

[ilyamisyura]

After updating to macOS 26 and thus migrating to Safari 26.0.1, Next/Payload website hosted on Vercel started to load extremely slowly (like 4.5 seconds). After short check, I've found

• __prerender_bypass and __next_preview_data cookies in request
• x-vercel-cache: BYPASS and Cache-Control: private, no-cache, no-store, max-age=0, must-revalidate in response.

At the same time, no such cookies and in general all good on:

• Chrome 141.0.7390.55, Edge, 141.0.3537.57 and Firefox 143.0.4 on the same OS (350-500 ms on the heaviest pages)
• Safari on another laptop with macOS 15.5
• Some browser on Linux on my friend's laptop
• Safari on iOS 18.6.2

Google PageSpeed insights shows 97 mobile and 100 desktop performance check.

My question to community: has anyone ever faced something similar? If yes, how have you mitigated that in an intelligent way?

Happy new macOS release! :'(

P.S. Meanwhile, I'm going YOLO mode and updating to iOS 26 Developer Beta to see if the behaviour is also present there. I'll update the post with new findings soon.

[ilyamisyura]

Well, the issue is rather stupid. Cleaning all the cache for the website solved everything. However, where did the production website get these cookies - a mystery that will be never solved.