The default setting is 4 and I doubled it to 8 and that delay is almost completely gone. On a newly opened instance of Waterfox it is almost as fast as if the page had already been visited in the same session.The explanation is "Number of data chunks we always preload ahead of read to speed up load of larger content like images. Currently size of one chunk is 256kB, and by default we preload 4 chunks – i.e. 1MB of data in advance" (from