I have the same low cpu usage problem as well. I have a fast AMD 1090T Hex cores CPU and 8G of RAM. But no matter what I tried, rendering to Sony AVCHD never get above 25% CPU usage. My work around is to open 3~4 Sony 10e instances then render them in parallel. That way CPU usages now can get up to 90~95% level. This is a tedious process but seems to be the only way to get things done faster.