Ok, new PC means Windows 10, which I have absolutely ZERO interest or knowledge in! Did you have any files transferred from your old PC to the new one? It's possible you could have given yourself a bad file already. However, I'd more readily think that your PC is backing up everything in real time to some cloud storage service. I won't use cloud! And one shouldn't, if they don't have a fast connection.
Do you have a security program? McAfee? Norton? Avast? Other? Norton doesn't slow my system down, but if I change the settings, it definitely would. This new Avast software....I don't like it! It has been slowing the PC down since I installed it, and they've tried to make it idiot-proof, which means I can't tweak it for best performance.
New modem? Are you going through a firewall? It's possible that your settings are not where they should be. Also, how do you connect to the modem - via cable or wi-fi? If using wi'fi, there are several factors that could be slowing down your connection. I know your literature may state that if you're within so-and-so feet of the modem that it'll work, but that's not worded well. The farther from the modem you are, the weaker your signal - hence, the slower your connection could be. Key word is "could".
Another question is, what speed was your old computer compared to the new one? What about RAM (memory)? The more RAM, the faster your PC can work. I would guess that this would be offset by the newer operating system, which should work much faster with the modern internet than your older system - but who knows!
As this is a "new" computer, I would hazard to guess that you have far too many programs running on there at any given time. Just like with smart phones, there are programs that you will likely never use that are booting up and running whether you use them or not. There are ways to go about thinning the list and culling out the needless software, but I'm not even gonna touch that one! If you're not 100% certain what a particular file or program is, leave it alone. It might just be essential!! Also, if you have security software that checks every single thing you do, then that could be causing a bottleneck.
Does the PC seem slow when offline? If not, then it's an online thing. Also, just for example, I don't use Flash on my PC. I removed it completely! I also have it so I can turn images off when my connection is really slow, plus having an ad blocker. So many sites now have these little videos that load automatically, and unless you have a high speed connection, you could sit there for hours waiting for all the stupid videos to load up! This is especially bad on many news sites.
...If you're connecting to your modem through wi-fi, you might want to consider getting a USB antennae to plug into the back of your PC. This PC isn't set up for wi-fi, but my antenna makes wi-fi possible. Also, not all antenna's are created equal!! I've bought/used three or four different ones, from tiny USB thumbnail-sized things to the one I'm using, here:
https://www.amazon.com/gp/product/B00BDE4IUK/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1
This thing is AWESOME!! I have a laptop that has wi-fi and bluetooth, but the built-in wi-fi is nothing compared to what this antenna can do! I also have another smaller antenna in another room that I definitely swear at more than swear by:
https://www.amazon.com/gp/product/B003YHYIT0/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
Hopefully some of these ideas will help.