If your computer can handle all the Java and updates, you're fine. If your computer can't it might get slower.
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No, not at all. It requires no Downloading.
Runescape could be slow because of: 1) Your computer doesn't support it 2) Your Internet connection could be weak or too many people on the Internet connection at once causing you to lag or 3) You could be on a world with to many people, when this happens you go to the lobby and choose a world with fewer people on it.
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.
Probably about 2 seconds, but if your computer is slow and internet is slow, it will lag and take up to 20 seconds, maybe even make the screen go black (this is also known as a game crash.)