I found these instructions in the Libreoffice Writter Guide:
Insert Picture dialog box
1) Click in the LibreOffice document where you want the image to appear.
2) Choose Insert > Picture > From File from the menu bar.
3) On the Insert Picture dialog box, navigate to the file to be inserted, and select it.
At the bottom of the dialog box are two options, Preview and Link. Select Preview to view
a thumbnail of the selected image on the right (as shown in the example), so you can verify
that you have the correct file. See “Inserting an image file” below for the use of Link.
4) Click Open.
If you followed these instructions, you have linked images into your document (a link to the souce of the image is placed into the document instead of a copy of the image). When it comes time to print, the souce files which contain the images must be opened before the linked image can be printed. That will certainly slow down the print process.
The other method of including images in text documents is embedding, which places a copy of the image into the document and should not slow down printing.