Not super great no, but enough to play the Orange Box and other games that scale relatively well at decent settings. Besides, putting it together is half the fun.
Thats negligible, especially if you have a fairly fast hard drive. I would compare it to PS2 load times, if that.
Don't know about you, but response has never been a problem with me, especially with mouse controls.
This does not happen if you know how to take care of your computer. Disable java, delete cookies, defrag with something other than Windows Defragger, don't use Norton, McAfee antiviruses, avoid IE like the plague, don't frequent suspicious porn sites, and you're set.