[derf_]

> i assume the variable bitrate uses less cpu, or?

At bitrates higher than about 64 kbps, CBR actually uses less CPU to encode than VBR. This is because the CELT layer was designed to be natively CBR (since for real-time communication you want strict rate control all the time), and VBR is achieved by varying the target CBR rate on a packet-by-packet basis (using additional analysis not required by CBR).

This was a major departure from codecs like Vorbis, which are natively VBR, and implement CBR by encoding at up to 16 different rates and picking the one closest to the actual target, or other codecs which use a "bit reservoir" to shift bits back and forth between frames (increasing delay due to increased buffering requirements).