If you have a good internet connection but still sometimes run into the issue of slow downloads on the PlayStation 4, this explanation by one tech-savvy Reddit user, MrRiddell, might be of interest to you. Do note, however, that his post is highly technical but there’s also a simplified explanation below for those who are curious. First, here’s what MrRiddell thinks happens after he carried out extensive tests:

The PS4 is (at least in some cases) using an incredibly small Receive Window which causes the download servers to have to spend a lot of time waiting, rather than sending more data.
During my testing, I was downloading for a server approximately 22ms away, so as a rough guide, the BDP calculation would indicate that I would need to be advertising a receive window of approximately 540KB in order to fully utilize my available bandwidth.
When looking at the TCP handshake from the PS4 I can see that it’s using an initial Window Size of 8192 (Bytes) and a Scaling Value of 6 (multiply by 64). All looking good so far and pretty typical.
PS4 SYN Packet
After the handshake has been completed, the Window size on the PS4 has been reduced all the way down to 135 Bytes which gives an effective window of 8640 (135*64). For the rest of the capture, the PS4 effective window fluctuates between around 3K and 8K but never exceeds 8640 Bytes.
PS4 Window Size
Because the PS4 advertises such a small Window, the download server can only send a handful of packets at a ...