By default this will encode the extras at Ultra Fast mode. This can be adjusted by one of the hidden commands. This is from one of the changelogs: Added a new hidden option "QUICK_USE_QUALITY=1" in which, when set, BD-RB will use the currently chosen quality level in quick extra encodes rather than the default of "--ultrafast" mode.
The quality of the encoding of the extras, when on ultra fast mode, will be less than what you see in the main movie. If the extras are not particularly important to you, but you still want to include them, this option will save some time and space at the cost of video quality.