Analysis: How much is the Bandwidth limiting the performance. A comparison between TB3 and TB2, between external and internal Display.  

In order to find out exactly how much is the connection bandwidth interfering with the performance, the following setup was used:

-Asus XG Station Pro with Geforce 1080 Ti
-15" MPB 2018 with 80cm Apple TB3 cable
-13" MPB 2015 with 100cm TB2 cable and Apple TB2->TB3 adapter
-Superposition benchmark, because it is very demanding for GPU compared to CPU.

The results yield following conclusions:

1) The more demanding the test, the less is the difference between TB3 and TB2 (external display). For smaller FPS the difference in bandwidth does not play a role at all. For 1440p Extreme there is hardly a difference between TB3 and TB2.

2) There is a upper limit in FPS when using TB2/3. This effect is clearly notable for TB2 as the score is not changing a lot between the 720p and 1080p tests. For TB3, which has a higher bandwidth the number are much more different.

3) It's best for an eGPU on external displays to use the highest settings possible. Most details and a highest resolution that are over 60 FPS.
In this test this would be 1440p High for eGPU with external Display.

4) For the loopback scores another factor is coming in: The resolution. As the data has be sent back again through Thunderbolt, the resolution plays an important role (other than for external displays).  That's why there is a notable difference in the 1440p test even though the FPS are quite low, but hardly no difference in the 720p Test.

5)For an eGPU on on internal display a medium resolution (1080p, FHD) with high details is recommended.
In this test this would be 1080p High for eGPU with internal Display (loopback).