The "default destination" for the ColorConvertOp is used when the user passes "null" as the DST parameter. In this case, the "ColorConvertOp.filter" creates the default image and uses ComponentColorModel- 8 bits per color component and optionally 8 bits for the alpha channel. For example for the ARGB source default destination will be RGBA format. Note that the RGBA format is not supported by BufferedImage directly, so the custom image type will be used.

Generic filtering of the CUSTOM image type is slow because we cannot pass that format directly to the CMM. It would be good to support CUSTOM images that were created by the ColorConvertOp.

Support of the default destination w/o alpha was implemented as part of the https://bugs.openjdk.org/browse/JDK-8005530. Since then we added support of the alpha to CMM code https://bugs.openjdk.org/browse/JDK-8012229.
Now we can improve the performance of the default destination if it has the alpha channel.