Alright, so grabbed a copy of the 7.5 Trial... ran it on an RX 480 and GTX 1060.
Ran each of the Sample Projects (on High Foliage, Super High Graphics, High Terrain).
Now something interesting is the RX 480 was seeing 21 - 36FPS on each Sample Scene., where-as the GTX 1060 was hitting 40 - 48 FPS. Yet, this actually isn't the only difference that I was seeing.
Lowering the settings on the RX 480, and basically they didn't make much of a difference., perhaps 10% at most improvement in performance... where-as the GTX 1060 was seeing quite a noticeable uplift through performance options being switch on/off.
In fact at the Lowest Settings 60 FPS was Possible... I mean I wouldn't recommend 50% Resolution / Lowest., but still it was possible.
On top of this the GTX 1060 was rendering the Scenes Correctly... in essence the RX 480 wasn't Rendering various approaches, Anti-Aliasing was entirely disabled on everything, Z-Depth Fighting was occurring on Alpha Textures as well as the Shadow Mapping (causing a nasty Chequer Board patterning, and again no Aliasing was being done).
I don't know if 8.0, 8.5 or 9.0 (as those are not the Trials they provided) improve this at all., but honestly given the Sales Pitch Video they had for 9.0 was essentially for showcasing Concepts to Customers in 'Real-Time' for Home Renovation / Building / etc. (likely drafted in Sketch Up using Lumia for "Fine Tuning" and "Vista Rendering") … I don't know, it wasn't exactly 'encouraging' especially as I can see it running like a dog even on a High-Performance Laptop.
In any case, the Engine (at least in 7.5) doesn't even feel like they've considered Radeon Graphics, let alone actually bothered to test it on such. Rather, it seems like they benchmarked it on the GTX 1060 and simply assumed "Well, if that can run it... so could Radeon" (ah if only it was that simple., make my life a damn sight easier)
Unfortunately I don't have an RX VEGA in-house to test with, at least not at the moment... essentially all the Engines that I develop in-house are targeted at Mainstream Graphics (so I have High / Low) Cards (in this case RX 480 / GTX 1060 and RX 460 / GTX 1050) and we simply scale within those; to hit performance targets.
Beyond that I do have a Pro Duo (Polaris) but eh., that basically ran it identically to the RX 480; so yeah was ignoring the Secondary GPU … at least until it crashed the Driver.
Although that's likely due to the Windows 10 Pre-Release., as it's a tad on the "Unstable" side.
Now, how to resolve the Performance Issues? Honestly, I'm not sure... like there's no obvious (again in 7.5) features that could be disabled via Registry Tags for the Application. Instead it just seems like a poorly optimised Engine, which doesn't really have a "Quick" Solution.
Rather what I'd recommend is keeping on Lumion' case about this., try to force them to better support AMD Graphics Hardware; as well, as far as I can tell right now; they're just not. The same is true for AMD Processors., as honestly something like the Ryzen 5 1600 should be overkill for an Application of this Nature.
Not sure what AMD could do to help with Drivers either., especially without Lumion themselves reaching out to work with their Engineers on optimisation.
