Thank you for contacting Aspose support.
I am afraid, it is always hard to pin down the cause when concerns are regarding the performance of the API. This is because the performance depends on a lot of factors including but not limited to the environment resources, API usage scenario & amount of data to be written. It is also possible that you may not have opted to use more efficient APIs in order to achieve the same goal while upgrading the project from 7.x.x to 8.x.x therefore it would be of great help if you can share an executable standalone sample application to demonstrate the problem. We will review it thoroughly to see if the code can we tweaked for efficiency otherwise, we can always request the product team to review the case to suggest improvements and/or tweak the internal model to get the best out of the API, however, we need the sample application along with supporting documents (input to the process) to start the investigation on our end.
Looking forward to your kind response.