At 720p48:
22:03:01.227: Output 'simple_stream': Total drawn frames: 82486 (101682 attempted)
22:03:01.227: Output 'simple_stream': Number of lagged frames due to rendering lag/stalls: 19196 (18.9%)
22:03:01.228: Video stopped, number of skipped frames due to encoding lag: 5362/101675 (5.3%)
Above your GPU is overloaded rendering the frame, and your CPU is overloaded attempting to compress the frame.
Less problem at 720p30
22:18:48.397: Output 'adv_stream': Total frames output: 3597
22:18:48.397: Output 'adv_stream': Total drawn frames: 3461 (3654 attempted)
22:18:48.397: Output 'adv_stream': Number of lagged frames due to rendering lag/stalls: 193 (5.3%)
Your CPU is ok here but your GPU is still a bit overloaded. I don't see a 1080p attempt but it does not look to me like your machine can handle it. You've got what looks like an older midrange i5 powerbook with intel graphics.
