I am advised VPN adds delay to network speed which affects iCloud sync. iCloud+ private relay also shows network speed; does it add any value when VPN is on?
Pretty much. Private Relay is kind of a “VPN-Lite” so with a full-fledged VPN, Private Relay isn’t really adding anything else except an additional hop for your internet traffic to make. It can definitely slow you down if you’re connected to a VPN server far away from where you are.
So with VPN, I should turn off private relay?
It doesn’t really matter. You may see faster speeds with private relay off.