Jerome,
Which cpufreq governor are you using when doing the experiment?
By default, we are using ondemand governor, so 70% cpuload doesn’t really tell us how busy the CPU is. We need to know at which speed it is running.
I recommend doing either
cpufreq-set -g powersave
or
cpufreq-set -g performance
it would set all CPU to run at the lowest or max speed. Then the CPU load will be meaningful.