Have a look at webpagetest.org, I ran a test on your website. It's telling me for the first request there is no time being spent on SSL negotiations, that it's time to first byte. That's typically processing time, if you can cache in Nginx for users who aren't logged in that could go do.
Strangely requests for JS files are taking time in SSL negotiation, whereas the other files aren't. I don't know why that is. Hopefully someone else can work that bit out.
GTMetrix says your page is well formed.
To speed things up you could put CloudFlare in front of the website, which may do some caching, depending on where the CF data centers are. They'll also do SSL for you, and they're pretty good at it.
Nginx page caching can also help, for users who aren't logged in it can be a big speedup. I have an article on that here.
It's not because of DNS, that's fast. It never helps to go faster. Route 53 would be faster, but costs money. CloudFlare has a free plan., and works fine with AWS. However Route 53 is a reasonable plan, but I wouldn't focus your attention there first.
It could also be because of latency, which you can't fix other than with a CDN. Where is the server relative to you - what's the ping time to it? I can see it's in AWS in ap-southeast-1 region - not sure which one that is.