Thanks for your inquiry. Aspose.Pdf for .NET supports msultithreaded environment so we would suggest you to please try using it. One Pdf document should be dealt in one thread, as access to a single document from several threads is not supported by Aspose products. Secondly, You may save the individual Pdf files into Stream objects and then concatenate the Pdf files. For further details, please check following documentation link for the details. Hopefully concatenating Pdf documents will help to improve performance.
Moreover, I'm afraid Aspose.Pdf don't have the feature of cloning Aspose.Pdf.Generator.pdf object.
Please feel free to contact us for any further assistance.