DirectX versioning and performance

So I feel like a bit of a noob for this, and I've been able to glean some of this information from various different threads I've found, but haven't been able to find anything definitive.
For this reason, I was hoping to reach out and get an answer or explanation on the DirectX differences. Mainly: is it expected that I would see better performance on DirectX11 Legacy over DirectX12?
If there's a KB article somewhere on this that I've missed I apologize in advance.

Context:
Started boxing retail again for the first time since Cata. Went through wizard, got to the DirectX part. See DirectX 11, DirectX 11 (Legacy), and DirectX 12 as options.
Choose DirectX12. Why the hell not, my limited millenial years of tech working knowledge has always lead me to believe to avoid anything with Legacy in the name (ironic).
Things work fine for a while. That is until I finally went to Org/Shatt/BC questing areas. Mount speed increases with flying and performance takes a nosedive. As a result my follow is breaking and I'm seeing excessive stuttering on various slaves.
Tried controlling from different characters, issue never really goes away. I'm very much the type of person to live with something annoying rather than spend half an hour and vastly improve my life, but I knew with my machine I shouldn't be experiencing these issues in Hellfire Peninsula or an empty Shattrath City with viewing distance set pretty low. If it was just Org, I'd suck it up and move on.

I started troubleshooting performance. In looking at RAM and processor, neither were pegged. Both sitting around 50% in Org during peak times.
This isn't my first rodeo as mentioned before, I already had max foreground and max background set to 30 and 15 respectively. I already run visual settings fairly low but haven't really optimized it at the same time.
At this point I know it has to be graphical so I started digging through configurations, options within my video drivers, and spelunking through the ISBOXER config. Then it comes to me, I chose DirectX 12, but is that really the right option? I mean, there's 3 of them. I found a thread on dual-boxing about how DirectX11 Legacy should be used over DirectX11, but nothing on DirectX12, and the thread was pretty specific to the issue that user was having. I switched to DirectX11 Legacy after reading that and immediately saw a performance increase.

Specs - built a few years ago as a low-mid range build for data processing and never really paid attention to specs, this is loosely what they are:
some AMD 8 core processor at ~3.6Ghz
32 GB RAM
AMD R9 380 (I think? It has 4GB of RAM)

TLDR: used DirectX12 because hurr durr it's higher version it must be better, see performance issues, revert to DirectX11 Legacy after some searching, see much better performance

[bob]

Directx12 is great. If you are not multiboxing.

You see performance issues because it requires 4 - 8x more VRAM than DirectX 11 Legacy. So your 4GB VRAM doesn't go very far when each game instance wants to chew up 3-4GB, vs the 512MB to 1GB of DX11 Legacy.

There are other overheads involved in new computer graphics tech, in that it does require more processing power (GHz) and if it doesn't have a hardware unit to perform certain tasks, then requires that to be done in software, which is more processing required. So your graphics card which was made with a DirectX11 hardware unit performs any DirectX12 operations in software, which is slower and requires processing grunt.

And there is a lack of experience for devs optimizing for DirectX12 because it is "new". Much more experience with DX11.

The "normal" DX11 in WoW, which his actually the new DX11, is the multithreaded version of the original (now the DirectX11 Legacy). The issue here is that the implementation is not the greatest, when multiboxing it is not very good at the hand off between instances. Essentially it ends up using too many threads!, which is rather ironic.

TLDR; DirectX11 Legacy is best for performance while multiboxing unless you have the latest and greatest hardware, and even then, you may need to limit the number of game instances you play.

This makes a lot of sense, thanks Bob!