Ich kills slowly if at all. It can go on for weeks, months or even years without killing a single fish. I have it right now in my tank and it is going on for at least 2 months. And it is not getting worse but rather slowly disappears as the fish become stronger and more resistant.
Even in the worst cases with severely compromised fish it usually takes at a minimum one to two weeks before it gets deadly.
If you have fish dying within days I rather suspect velvet, brook (though that usually shows pretty clear symptoms) or some toxicity.