Update: We have discovered that force iCloud private relay back to TCP mode (and setting back the MTU of a Mac to default/1500-byte) signifcantly improved the performance of Safari browsing, including Youtube videos. We wrote a single PF rule on a FreeBSD LAN gateway to block UDP packets to port 443 and echo ICMP to the Mac.

If it’s the latter, the Apple Developer Forums isn’t the right place for this question I am sorry but I have to post the thread here as I am using macOS 27 Beta.