Poor Performance x264

[kenshin3it]

Hi everybody,
i've downloaded Avidemux 2.6.5 and when i try to encode a Standard Definition clip with x264 ....it's a pain.
i'm setting x264 to be compliant with Main Profile @Level 3.0 and i obtain a speed of 17fps.
When i use Avidemux compiled by Homebrew i obtain a speed of 90 or 100 fps.
Can anyone help me to understand why?
My system is an iMac 21.5" late 2012 (Core i5 Quadcore and 8GB RAM).
Thank you very much.