From the first deployment, I have not modified anything over half year.
Today I found when I opened the website, the homepage did appear slowly, and I was waiting for Google fonts. Another issue caused the slow response was the access of some avatar websites.
Then I installed two plugins to prohibit the above websites, but when I tried to enable them, I got an error that I must install PHP 5.4 or later. This was not a big problem, so I installed PHP 5.4 from remi repo, and it turned out to be not the same easy as I though.
When I wanted to open the homepage, it just gave me a blank page. I checked the log of webserver and found the memcached could not worked with the new PHP. From the error message, I searched the internet and got an answer from W3 Total Cache plugin forum -- reset it to default settings. It did work but I still got some issues about the PHP and Nginx. I spent half day to tune this website and eventually got a satisfying result.
One day, I hope I have enough time to perfect the appearance of this website.