Large PowerPoint files usually extend the time it takes to build a Book by a decent amount. It's possible to circumvent this issue by making sure your PowerPoint file is optimized. 

Here are a few things you can do while working on the PowerPoint, and after completing it to help reduce the over-all size of the file.

While Building the PowerPoint:

• Insert files, avoid drag and dropping or copy and pasting files into the PowerPoint.
  • Files that are added to the PowerPoint by using the Insert feature tend to be smaller in size than files added by other methods.
• Edit Images in an external Image Editor, and then Import it into PowerPoint.
  • PowerPoint saves every stage of an image you edit inside of the PowerPoint (color change, resize, etc) that adds a lot of data to your PowerPoint file.
• Avoid Embedded and External fonts. 
  • Removing embedded and external fonts tends to reduce the size of the file by a noticeable amount.
  • To change this setting, click on File (located in the top left corner of your PowerPoint), and then select Options in the far bottom left. Then you'll want to select Save on the left side of the new window. Finally, make sure to uncheck the 'Embed fonts in this file' option. The settings should now match the highlighted section of the following image: 
    

After Building the PowerPoint:

• Save your PowerPoint as a .pptx.
  • Saving your PowerPoint as a .pptx file will automatically compress a large potion of your presentation. Most PowerPoint files are saved in this mode by default. If your file is not in this mode, you can change it simply by clicking on File (located in the top left corner of your PowerPoint), and then selecting the Convert button. It should look similar to this image: 
    
• Compress all your images. 
  • Compressing your images can dramatically reduce the size of your PowerPoint. To do so simply click on File (located in the top left corner of your PowerPoint), and then selecting the Save As option. In the new window click on Tools, located in the bottom right, and select Compress Pictures. 
  • The options that most reduce the file size of the PowerPoint are as follows:
    • Delete cropped areas of pictures.
    • E-mail.
  • The settings should look as follows if you selected the maximum file size reduction:
    
    
  • Note: you can select other Resolutions if you'd like to preserve a higher quality image.
• Save PowerPoint Presentation as a new file.
  • Saving the PowerPoint as a new file would remove any copies of the PowerPoint attached to the file due to the AutoRecover feature.