I actually found that GitHub Pages didn't scale well at all -- during peak traffic, response times slowed to over 10 seconds. This was my main motivation for the switch. Nginx (even on a micro EC2) should be able to handle very high load without the need to scale. S3 is definitely a viable option as well (and I'm not suggesting you shouldn't use it) but I find that an EC2 was a better fit for my needs.
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I actually found that GitHub Pages didn't scale well at all -- during peak traffic, response times slowed to over 10 seconds. This was my main motivation for the switch. Nginx (even on a micro EC2) should be able to handle very high load without the need to scale. S3 is definitely a viable option as well (and I'm not suggesting you shouldn't use it) but I find that an EC2 was a better fit for my needs.