Is there a way to revert to previous User Interface while maintaining full functionality?

No way of reverting back personally I haven't experienced a loss in speed or performance with UI since the update if anything it's quicker.

Could try rebuilding the PS4's database via the safe mode to see if this speeds the OS up a bit.

https://www.playstation.com/en-gb/get-help/help-library/system---hardware/power---system-stability/safe-mode-on-playstation-4.ps4/

Although other devices are working fine I wouldn't rule out a connection issue,wired connection will always be vastly superior over a wireless connection.Router firewalls can be another factor opening ports or setting up a DMZ can often resolve this.

http://community.eu.playstation.com/t5/Gaming-Network-Support/How-to-put-your-PS3-or-PS4-into-a-DMZ/td-p/10655314

http://manuals.playstation.net/document/gb/ps4/settings/nw_test.html