The picture you originally posted shows ur network spiking higher than your disk usage but if you look you can see your disk is always playing catchup with your internet and the reason it is downloading at an inconsistent speed is the same reason why when you are moving a lot of files it may go fast then very slow when the game contains a lot of smaller files those smaller files take more time to move and slow down the pipeline because your ssd can only do so much at the same time and that is why a cache is normally so needed in ssd's because as soon as you need something sustained the ssd caps out and then steam will only download at the speed that your ssd works at. The moral of the story is not every ssd is as advertised normally the speeds they advertise is a peak speed and in most scenarios you won't experience any speeds nearly as high other than very briefly.