There have been instances where people have said that WinRAR doesn't work properly on their comps, whereas 7Zip does. They both basically work the same; the only real difference between them is that 7Zip is freeware. I personally have and use both on my computer. WinRAR is my default application, and if there's ever anything that I get that it cannot handle, then 7Zip is my backup.
And to address your original concerns, neither WinRAR nor 7Zip have any effect on your DL speeds.
I use winrar all the time and it doesn't slow down my internet or anything. I don't know whats going on with your computer Sentinel King. That is strange indeed.