Superfetch randomly downloads/uploads data because windows wants to "Maintain" and "Improve" without you knowing so.Superfetch preloads files from disk into memory, because it thinks you will soon use them (it can detect for example that you open your mail app every morning, and many other patterns, simple or complex).
So Superfetch only affects disk performance. However it's usually a bad idea to disable it, because you will have to load these files anyway, resulting in delays and disk and memory usage bursts rather than loading things progressively.
Some cases may train it in a wrong way, making it load lots of useless stuff. It may be the case here, considering the enormous amounts of files osu! uses, and the fact they almost change on every use (except if you're always playing the same beatmaps and not scrolling through the list, making it not load other background previews).
I hope that also answer your question, zharga_Lolz.
The services mainly involved in downloading for "improvement" are BITS (Background Intelligent Transfer Service) and Delivery Optimization.
By the way CPU interrupts often have close to no effects (but sometime do) on CPU performance. Rather than using a network sniffer (there is too much noise, a newcomer can't find anything with that), look in "Processes" tab of your Task Manager for the line "System Interrupts". If it's under about 5%, it's certainly not the cause of lags.