After looking at the samba configuration file I saw that Async IO ('''AIO''') was not enabled by Amahi. What AIO does is that it let's Samba create multiple threads to handle file operations. This can, at the expense of some CPU cycles, increase the performance quite a lot.
I increased the read speed to 500-600MBit/sec by enabling AIO and saw Samba using around 1-4% CPU on a Intel E2200.
==Enable AIO==
'''It can not be under stressed. Bad options can be very BAD. It can corrupt your data and make you very sad. Use extreme caution if elaborating with the options!'''
If you add a new share in Amahi or change the settings of an existing one, Amahi will overwrite the '''/etc/samba/smb.conf''' file with the new configuration and thus removing your changes. A place to enable '''AIO''' in the settings section of Amahi has been suggested athttp://bugs.amahi.org/issues/show/861