The .NET framework supplies the infrastructure your machines need to run .NET programs.
if you are not running .NET progs then you don’t need it BUT so much stuff from Microsoft is .NET based that you will eventually need it.
Just installing the framework will not slow down your machine.
There have been a number of service packs and upgrades for .NET. if you are going to install it my ecommendation is to install .NET 3.5 SP1 which is the latest version. patches can be delivered via Windows Updates