I know it is too late, but I write this answer for those folks who are struggling to solve this problem. Recently, I have done a rendering project using InteropBitmap in which I was able to run about 16 media player components in a WPF window at the same time on a core i7 1.6 Ghz CPU +8Gb RAM Laptop, with 25fps. Here are the tips I took for performance tweaking:
First of all, I did not let GC handle my video packets. I Allocated memory using Marshal.AllocateHGlobal wherever I needed to instantiate a video frame and Disposed using Marshal.FreeHGlobal as soon as I did the rendering.
Secondly, I created a dispatcher thread for each individual media player. For more information, read "https://blogs.msdn.microsoft.com/dwayneneed/2007/04/26/multithreaded-ui-hostvisual/".
Thirdly, for aspect ratio, and generally the resizing purposes, I used native EmguCV library. This library helped me a lot on performance rather than using bitmaps and overlays and etc.
I think these steps help everyone that needs to render manually using InteropBitmap or etc.