I found out that an imagemagick/convert application on my VPS is causing troubles due to high load on my box. Now i added renice with priority 10 to all of these convert-processes for throttling each minute.
However, I am not sure if it really slows down the process and relieves my CPU (only 1-core).
How to really make sure that specific processes do not increase the load significantly or attack the stable system?
I have >500MB free RAM, hardly any swap in use and a low iowait at any time. So I guess, it's really because of the cpu with only 1 core. Most of the time, this is enough and no waiting times. But in case of some conversions are running, it's failing.