Efficient way to create video from looped .gif file?

Question

I have two files: animation.gif and audio.mp3; duration(audio.mp3) >> duration(animation.gif). I want to create .webm file from the two, i.e. looped (unknown number of times) animation.gif + audio.mp3.

A simple (or, at least, obvious), but slow way to create that is:

ffmpeg -ignore_loop 0 -i animation.gif -i audio.mp3 -shortest -c:v libvpx -threads 4 -b:v 400k -f webm out.webm

So, the question: are there any tricks to speed that up? Is there a faster way to create a long video from looped animation.gif?