You are after the -threads option of avconv. Safest setting would be:

 -threads auto

but you can also set an integer there if you want to experiment a little. A modern FFmpeg (now the standard in Ubuntu) sets this is auto by default as seen in this section of the 'ffmpeg-all' man pages:

threads integer (decoding/encoding,video)



Set the number of threads to be used, in case the selected

codec implementation supports multi-threading.



  Possible values:



     auto, 0

     automatically select the number of threads to set



 Default value is auto.

Note 2 important points:

1. This setting will only have effect if the chosen codec supports multi-threading


2. It is possible to set the thread count for individual streams rather than simply attempt a global thread setting