Progressive rendering of HEVC is not possible in the same way as it is done for JPEG or PNG. It is also not very suited to GPU rendering.
A C++ decoder with assembly optimizations could easily run four times as fast as the Javascript version. Unfortunately distributing it would not be possible due to patents.
There is also the overhead of using a canvas element as a render target instead of a normal image surface, and using the canvas API instead of just writing color values straight to a buffer.
A C++ decoder with assembly optimizations could easily run four times as fast as the Javascript version. Unfortunately distributing it would not be possible due to patents.