Xbox one x showa 400mbs wired/wireless but downloads and general internet access from it is in kbps....Fortnite taking 10 hours to update. Reset router, reset Xbox, test the lines, turned off all equipment for 30 mins, pc updates Fortnite etc at normal speed as does PS4 etc..Xbox one only one having issues that were not there yesterday.
It will be Microsoft’s servers limiting bandwidth.
Try power cycling the console or pausing and resuming the update after a few minutes and you might get load-balanced into a better download server.
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