If you are facing system slowness issue, that means there are some process or programs are using your internet connection.
For example, a program has a automated process to do the updates.
I would recommend to check all your installed programs, and see if you can disable their automatic updates.
First step to do, and try to make sure to disable your windows automatic updates, and see if the computer works any better.
If it does not get better with automatic updates turned off, check the programs updates then.
To disable some services, take this steps:
3. type: msconfig
At this tab, disable those who have to be with updates.
But ATTENTION: You must NOT disable some system services, since it may cause problems with your windows.
If none of this works, I would consider formatting the computer.