Hello,
Thanks for your new package. we have tried this at our end and seems to not having major break through.
Having the same behavior and as Spire is writing every thing into memory while processing and then saving to disk is causing Memory problem.
The test link I have shared with you (https://www.pdf-tools.com/pdf/pdf-merge-split-files-manipulate.aspx), is much faster then what you have provided.
Not seeing any significant difference in problem behavior.
Please let us know if require any other information.