Why does write/read speeds drop drastically?

[gr8pics]

I got a imac 27 late 2015, got a 2TB fusion drive, 1.5TB free space.
Using Disk speed test, loading file 5gb, and runs continuously, it shows 600/1500 once or twice, but 9 out of 10 times it slows down to 160/160
Can someone explain why?Im running the test from the root of user folder.

OSX 10.12.4
[doublepost=1493314451][/doublepost]Can moderator please delete the 2 most recent duplicates of this post?

 

[shaunp]

At a guess I'd say it slows down as it needs to move stuff from SSD to HDD as part of the fusion process.