While converting a PDPage to a BufferedImage (using the PDFRenderer) Images-Resources located in the PDPage gets loaded in a WriteableRaster and a given Colorspace.
These get converted to a BufferedImage (with PDDeviceRGB.toRGBImage()). But the Type of the returing BufferedImage is with the given Raster and Colorspace always from Type "BufferedImage.TYPE_CUSTOM" instead of the expected "BufferedImage.TYPE_INT_RGB" for example.
Image-Processing-Operations with/on BufferedImages with TYPE_CUSTOM are not optimized in Java (slower) and partialy runs through "Synchronized" Blocks.
==> Converting different Documents in different Threads are blocking each other.
"Quick"-Fix for seeing a Performance-Improvement in a Multithreading Environment: