Using External BMD GPU doubling export times.

Today I had a chance to do something I've wanted to do for a while and test practical exports with the Apple official BMD eGPU (the original) and noticed some interesting times.

I have a 2017 iMac 3.4 i5 64 GB and rendering a 4k 25p timeline of just over 8:30.
Anytime I specify the eGPU my rendering of clips, and few simple graphics titles (no heavy animation) Im getting the following times:

Same export setting used for all exports (a compressor export I set up in FCP)
eGPU 580 using FCP- 21:22
eGPU 580 using Compressor 20:44
internal 570 using FCP- 9:18
internal 570 using Compressor 10:10

All other conditions are the same except for physically disconnecting the eGPU.
Anyone getting other results like this?
In earlier versions I believe I was getting better times.

my export is a h.264 for final delivery- did not test other codecs yet.

I can not say for sure but perhaps when you use the eGPU FCPX might not use Intel's Quick Sync. Can you run a statistics/resource benchmark program?