I was seeing a performance issue with Safari displaying pages (literally taking 10's of seconds for any new page) when Private Relay was enabled. I discovered the problem went away if I enabled a VPN (PureVPN), suspected a ISP DNS problem but changing to another DNS server did nothing to help. What did get around the problem was disabling AdGuard before starting Safari, now performance is normal with or without a VPN enabled.
Switched to using "AdBlock for Safari" and all is good, performance normal and ads blocked!