You can achieve no delay without using a zero delay. Graphic Control Extension blocks may be omitted between images and so multiple images can come in a row without anything defining a delay between them.

See the last instance of the phrase "graphic control" in the document to see that multiple images can come with no intervening GCE blocks and see the paragraph containing the 5th instance of that phrase to observe that a GCE block only affects the first image following the GCE.