No, not at all. It requires no Downloading.
Chat with our AI personalities
If your computer can handle all the Java and updates, you're fine. If your computer can't it might get slower.
If Minecraft is not running, no it will not slow your computer down.If you have a low end or older computer then playing Minecraft may slow it down a little as it's a fairly hardware intensive game. Just give it as much ram as you can and turn the graphic details down until your happy with it.The Minecraft WiKi has a lot of information on managing the game.
It depends. As a ROBLOX player myself, it has not done much. But, if you have a old, slow, or/and defective computer it may.
It can slow it down, but it doesn't do any major damage like spyware or a virus.
It will while the program is running, but after it stops running there will be no change in computer speed.