Thanks for your interest in Aspose.Slides.
I have observed your comments and like to share that the time taken during cloning slide depends on source presentations sizes, number of slides and contents inside slide especially multimedia content. In your case, you are trying to load 20 presentation files and merging their individual slides to form a single presentation. I like to share that loading 20 presentations is certainly going to consume more than 3-5 seconds even if they are simple presentations with minimal data. There is no definite time that can be shared as it is dependent on factors that I have shared with you. Please share, if I may help you further in this regard.