You could do a speedtest to veryify your connection.
But it's probably PreSonus' server.
Some of the files you mention are very large to huge in size and will take awhile to download. It's not like you're going to get them all in 2 or 3 minutes. Have patience.

It depends where you are. This has been mentioned before. They load real quick where I am. Every time. Although I don't download them through Studio One though. I go directly into my account (with Studio One closed) and download them from there. Once downloaded all you have to do is drag them onto the start page and they will install. It is not a bad idea to back them up either. On a drive or burn some DVD's. Then you won't have to do it again.

Do a speed test. There is probably a guy next door who is super gamer and he is eating up the bandwidth. I am on fast connection too but every time I do a speed test I get a different result. Different times of day as well.

Also don't just take one speed test for gospel. I am on a connection that goes up to 100 Mbit/sec but I am only paying for about 35 Mbit/sec. Mainly because I am pretty happy with that speed. One speed test shows my speed at 8 Mbit/sec but another is around 30 MBit/sec. It certainly feels like the higher speed to me that is for sure. Do multiple speed tests. The slower one is obviously wrong.

I'm having the same problem downloading my upgrade. This has been an ongoing problem and a google search reveals that it's been going on for years. I've tried shutting down my AV software, ran a speed test with download speeds at 30 Mpbs, so it's not the network. It's not my hard drive because there's plenty of space for the program. This is something within the software program itself doing the upgrade by the sounds of it. It's coming in drips and dribbles with the network monitor spiking, then nothing, then spiking, then nothing. I've read that Presonus has their servers on a bottle necked network. I'll try downloading and installing manually as suggested above. Hopefully, that's more efficient. At this rate, 18 kbps, XT Acoustic Guitars is going to take 24 hours to download. Not acceptable.

Update: Still slow, but these are huge files. It's much, much faster downloading one at a time from within your account than from within Studio One.

Total download time on high speed cable connection was three hours. This was upgrading from Artist to Pro.