If you use "domain user account", to run your sql server, who does not have requisite permissions to register SPNs. You will get that message but I believe that is not the reson why your server is slow.
Has anything on the network infrastructure side changed? New nics or switches? Makesure both the nic and port on the switch are configured identically. ie configure the nic for 100 full duplex and the switch port for 100 full duplex.
I've experienced sudden performance drops when the network crew put in a new switch. The nic was configured 100 full duplex and the switch port for auto negotiate. This was a problem and was fixed by setting the switch port to 100 full duplex.
There could be many other things which would do this.