Thanks for your patience.
We have further tested the scenario with latest version Aspose.Pdf for .NET 17.11 while using following code snippet and thoroughly inspected our source code. We have not found any block that could be simply and essentially optimized. Please note that conversion duration naturally depends on number of elements on the page, especially characters. As you can see, this document is composed quite densely, so performance with this sort of document is expected and acceptable.
In case of any further assistance, please feel free to let us know.