Same issue on Xbox. Probably a latency issue. Mine is awful.
Their file system as well as how they manage files always contributed to slowing the process down, and it seems they aggressively meter/shape PSN transfers as well.
The PS3/4 also had some weirdness in their network stacks that created weirdness as well.
Some people would get around some of it by using a proxy to a local pc/laptop, or would set static MTU's lower than what the auto setting would detect. In TMO's case, that needs to come in at 1420 or less or there WILL be fragmentation.
If you have the option of a router based VPN, it could potential help as well... all depends if you are getting shaped closer to home or not (which seems to happen with TMO during high usage time frames).
I didn't mean to flag this topic as answered.. how can I "un-answered" it??
Also, I should add that the PSN downloads are slow even before the 50 GB throttle mark and I should also mention that even after the 50 gig mark I don't notice any throttling of speeds or quality in my area, at least not yet..
There isn't a line drawn in the sand for how much you consune. It is a prioritization scheme.
Phones are higher priority.
Home internet is deeply deprio'ed.
When usage ramps up and starts pegging high utilization, home internet packets are the first to get stuck in queues.