What is the best way to GetPixel out of a RenderTexture?

I'm trying to get a fluid simulation working on Android.

• Running in C# is too slow.

• Making a C++ plugin could potentially be faster. I've never written such a plugin, but I do know C++.

• Compute Shaders are out of the question because I wouldn't even be able to test on my phone (Nexus 5).

• I have a working simulation that uses regular Shaders and RenderTextures, but they don't have a GetPixel function which I need to be able to have the fluid push my GameObjects around.

The best solution I've been able to find so far is to make a Texture2D and use ReadPixels to fill it, then GetPixel out of it. The code I'm using is simply:

 RenderTexture.active = m_velocityTex;// The RenderTexture.
 velocityReader.ReadPixels(new Rect(0, 0, m_width, m_height), 0, 0);// The Texture2D.

I've tried having that code at the end of Update, in LateUpdate, and in a coroutine that uses WaitForEndOfFrame, as well as a coroutine that manually tells the camera to render then waits a frame before the ReadPixels call.

But no matter what I try, ReadPixels kills the frame rate. Is there some other way of reading from a RenderTexture or setting it up so that ReadPixels doesn't cock block the main thread?

Also, can anyone comment on whether it would be worth trying to port the sim into a C++ plugin?