Adding pictures into a graph was very slow. The cause of slow work has been now discovered. I have already reported that small chunks of data were at the heart of the problem. Now I seems to be not that large disturbance compared to what has become known.
The number of operations to put a pixel into the graph was tremendous. It was checked at each dot of the raster whether the dot already existed. All target vertices of a height value and all target vertices of a width value were fetched to find a common target. Both values went above 1000. This gave a thousand checks at each of more than a million dots.
So, I decided to run initialization at the start and then do no more checks. The vertex IDs will be kept in a 2-dimentional array.
I hope to get better measurements in JVM-version as well.