Thanks for inquiring Aspose.Slides.
I have observed the information shared by you and request you to please share the source presentation, generated presentation, issue details ans used sample code.
For your question concerning to saving presentation in PPT or PPTX is concerned, I like to share that PPT is stored in binary format where as PPTX is stored in OOXML format. Because PPTX and PPT have a different format for storing information (PPTX is zipped XML and PPTis binary format), the conversion of information to store it in these different formats differs in its logic, which affects the time and give a little edge to PPT in terms of time while saving presentation.
Please share the requested information so that we may proceed further to help you.
It’s good to know that things are solved on your end. Please share, if I may help you further in this regard.