I have a 6 month old top of the line iMac. I always can tell immediately if Time Machine is running b/c the computer becomes choppy. This lasts for the duration of the backup. It is most noticeable when I move my mouse. The mouse starts to act jumpy rather than gliding. In activity monitor it shows during the backup that I still have 50-80% idle so I don't understand why I take such a performance hit. I do backup wirelessly to a Time Capsule. Is this the issue? It is often backing up large amounts (though I'm not quite sure why). I'll download about 100MB of pictures and it will do an 800MB backup. I just can't shake the feeling that something isn't correct. I've noticed this back on regular Leopard and now on Snow Leopard. Apart from this issue my iMac works perfectly. Please let me know if others see this or if you have any ideas what the problem is. Also, if I can't fix this, is there anyway to slow down Time Machine backups (without hacking type stuff) so it only runs once or twice a day? I really don't need hourly backups. Thanks.