In my experience using AWS, you can spin up new autoscaled instances pretty quickly. I don't even have a custom AMI, I use a generic image and run startup scripts to kick off my server software. Takes < 3 minutes to spin up a new one and start accepting requests.
If that's not fast enough, you can bake an AMI and scale more proactively (e.g. scale when your server is 70% utilized rather than 95%)
If that's not fast enough, you can bake an AMI and scale more proactively (e.g. scale when your server is 70% utilized rather than 95%)