this can be fixed by turning it off by changing

> Modules/Core/src/IO/mitkItkImageIO.cpp:      m_ImageIO->UseCompressionOn();

to

> Modules/Core/src/IO/mitkItkImageIO.cpp:      m_ImageIO->UseCompressionOff();

this reduces save times by literally >99% and does barely increases most files in size (e.g. +<10%)
but of cause an option to do this nicely would be more ... nice