The processes that run in the background form jus a small part of the real thing. The internet could be slowed down due to many factors, like connectivity, bandwidth capping. some sort of infection on the pc or some update process running in the background. In order to be sure what's what, u can follow this to know and learn about windows services and processes.
The above mentioned site will help you understand services in the o/s.
The best way to deal with slowing down of the internet is to understand the nature of the slowdown, like i've mentioned above and sometimes a firewall does come in to good use just to help understand what's happening.
If you just want to monitor, your bandwidth consumption and which processes are connected to the internet then DU Meter is the best. But if you want to limit some processes and such go for NetLimiter.
simple way to put it, Net Limiter is like an advanced version of DU Meter.