I tested using the default settings of radeon pro render for blender with some exception of some settings like the number of samples that i changed to 128 istead of 64 in all tests and samples updates per view in some tests
so lets begin
test 1= cpu+gpu/samples per view update:4 = total time 01m.11s.19ms
test 2= cpu+gpu/samples per view update:64 = total time 04m.26s.07ms
test 3= gpu only /samples per view update:4 = total time 00m.18s.58ms
test 4= gpu only/samples per view update:64 = total time 00m.16s.09ms
and pc specs
Os: windows 10 1809 compilation 18362.239
Motherboard: asus asus m5a78l-m br usb3
ram: 8gb Hyperx 1600Mhz
Cpu: amd Fx 6300
Gpu: Asus Radeon Rx 460 dual 2gb
Gpu Driver version: adrenalin 19.7.1
Radeon Pro Render Plugin For blender 2.80 version 2.0.112
Blender 2.80