MacPro after wake-up takes ages before Wi-Fi connects?
Did you try to delete your wifi settings? 


Go go to System Preferences > Network > Use the (-) to delete wifi. Use the (+) to add a new wifi setting.
Do you use additional network settings e.g. Ethernet or iPhone USB? If so, try to disable them or set the Service Order accordingly.
It could be a network issue. Maybe your wifi DHCP Server is slowing things done.