We apologize for the delayed response.
We would definitely try our best to reply you within the time that suits you. Regretfully, forums are the only station where we encourage providing support.
Nevertheless, we have tested the scenario using Aspose.PDF for .NET 20.1 and observed that the API was taking long time while saving the PDF document into MemoryStream. The issue looks related to large memory consumption. You may please try saving the PDF to local disk instead. However, we have logged a quality issue as PDFNET-47632 in our issue tracking system.
We will surely look into details of this issue which is related to the performance of the API and keep you posted with the status of its correction. We request you to be patient and spare us some time.
We are sorry for the inconvenience.