There are a whole bunch of possibilities, but you can at least do some basic tests to see where the problem is most likely.
Some thoughts....
Firstly, there is nothing wrong with 8Gb RAM, but the important thing to remember is that the more RAM, the more apps can be open at any given time without slowing the system, so try using the system with the fewest apps/windows open. Also, as macOS has been updated, it gets bigger and bigger, so uses more and more RAM just for itself.
Wifi and internet speeds can be tested reasonably easily with online speed tests, and you might have two possible issues: firstly your internet service, if DSL will be very slow anyway, and if cable you may be suffering 'choking' just because of the amount of data traffic on the node that provides service to you (there can be a large number of different customers sharing a single node, which is why cable is usually quote as '...up to...' speeds). Secondly, your wifi access point/router may be running slowly. This can happen if it's relatively old, or if there are a number of others using it.
1Tb fusion drives have relatively small SSD components in comparison to 2 and 3 Tb units, but while they are not as data efficient as the larger drives, are usually sufficient unless the drive itself is getting very full. The 1Tb drives do have a reputation for relatively slow performance compared to the larger drives but this would normally only be particularly notable if you were doing a lot of large-file saves, or running a large number of apps at the same time - this latter is important because like other systems, Macs use the hard drive as temporary storage, known as 'caching' or 'paging', which happens more and more as more apps are open and less memory is available.
Even if the SSD part of the fusion drive isn't enough, the rest of the drive is a standard 7200rpm unit, which for normal purposes isn't all that bad in performance, but can hit the system badly with caching/paging because the hard drive is a great deal slower than the system's in-built RAM.
One point I think you made was that you saw a lot of speed issues when accessing iCloud - beachballing. While that could be a drive issue, it likely isn't because even a slow laptop drive can throughput more data than a typical wifi+cable internet, which means that I would doubt the drive is the sole culprit.
If I owned a Mac that was performing as seemingly badly as this, I would back it up using (preferably SuperDuper or CarbonCopyCloner) and then restore it to 'as new', before reinstalling apps and data form the backup. If nothing else, this would help remove the possibility that macOS has been compromised by data errors.
