When updating my download speed randomly drops?

Hey, I was just trying to update fortnite yet as I am doing so my download speed randomly drops to 0 for a while then goes back up.I have done a speed test
Download : 35mb/s
Upload : 16mb/s
Service provider : BT
Location : England, London

Anyone know why ?
if so please say,

For Fortnite related issues head over to Fortnite forums.

Judging by your description, it does sound like their servers might be a tad under pressure.