Question about performence of AMD cards  

hello everyone, I'm back

now the policy of apple is not friendly to nvidia users

i feel there is little possibility  to put 2080ti in eGPU enclosure for rMBP

so I'm considering using a Radeon VII instead of 1080ti as an eGPU card in order to move on

1080ti do have strong calculating power, but it still has inconvenience like must shut down rmbp before disconnect the eGPU, can not update macOS above 10.13.6

but last year i got a vega64, and i'v run some tests via Davinci Resolve and Premiere Pro, the result is quite ... weird

e.g. here is a grading result of a TV program:
rMBP 2017 15'  w/ macOS 10.13.6, Razer core/changed to core v2 later/ the program lasts 46-48min, moderate load with denoise, prores 422HQ footage

mostly i use EVGA 1080ti FTW3 for grading and rendering, one episode may take less than 40 minutes to render (CUDA)
before i got the 1080ti, i've used an old Titan X (maxwell maybe?) and the time should be around 70 minutes (CUDA)
but when i tried Vega64, i spent over 90 minutes to finish rendering (as tested AUTO is the fastest)
i tried internal Radeon Pro 560, .......it took me 2 hours and 42 minutes and i don't want to try it even one more time

in recent tests via vega64/1080ti/2080ti indicates that when installed in a pc, the familiar projects
render time of vega64 is quite closer to 1080ti then eGPU in mac does
i tried render 10min of program and got these:
macOS 10.14.4/core v2/PC win1o, result in minutes
OpenCL                            42: 01          
METAL                             14:49                  
OpenCL(PC)                     6:24                            
OpenCL(PC,external)     9:59
*PC has a pci-tb3 card and tested with the very same enclosure
**BTW 1080ti/PC/internal: 7:33       2080ti/PC/internal: 3:09

don't u think rendering 10 minutes of contents would take 42 minutes is INSANE?

so I'm here to ask if someone could explain this, thanks

and if it is necessary  to replace the 1080ti with a Radeon VII

The biggest question is if radeon vii can provide the same level of computing power to 1080ti, in an eGPU enclosure

====================================================
Personally Ideas of Conjecture
-CUDA efficiency
-OpenCL needs very strong CPU helping encode/decode/compute
-AMD card has extremely high bandwidth witch thunderbolt can not provide
-Structure or black technology of graphic cards/cores
-Drivers
-The vega64 i got is abnormal
-Power should not be the problem because vega64/ext with PC is tested and way more faster than the very same set with mac

Thanks for reading, i'm considering to buy a Radeon VII for test myself if no conclusion could be made here