SIMD, you say? Are you relying mainly on NEON optimizations or are you also doing encoding stuff on the GPU? Very impressive performance I must say!
I first tried using the GPU, using old school GPGPU textures and OpenGL ES 2.0 shaders, but unfortunately the performance wasn't there for a variety of reasons given in [1].
SnappyCam has since been making extensive use of ARM NEON for the JPEG codec and a bunch of other image signal processing operations, like digital zoom. It's a great instruction set!
[1] http://www.snappylabs.com/blog/snappycam/2013/07/31/iphone-k...
It's like MMX / SSE of x86 world, a set of extra instructions to process many small integers in parallel in one instruction. Since image data are usually independent 3-byte pixels (or 3 planes of 1-byte subpixels, one per color channel), NEON is great for many image-processing tasks.
See e.g. https://en.wikipedia.org/wiki/ARM_architecture#Advanced_SIMD...