In certain versions of ImageMagick there is a problem (probably due to a bug) when trying to convert or manipulate an image either from the command line (convert command) or through one of the many API’s available (for example PHP). The CPU usage suddenly grows beyond 100% (because this problem occurs on multiprocessor systems) and the system becomes extremely slow. Apart from this high CPU load, the conversion process also gets blocked, never reaching to the end. This occurs even with small images of few kilobytes.
After some research, I indeed concluded that it’s a bug present in certain versions of ImageMagick compiled with the –enable-openmp parameter running on machines with multiple processors or multiple cores.