Not enough information
But, I will make a guess that people receive a timed out message.
The total size of the page exceeds 3.5 mega bytes in 73 files. With high speed connections, the total size per se is not a fatal error, though slow connections will make it so.
The css files total 7 times the size of a reasonable amount for even a large and complex site. Specifically, these file are ridiculously large:
https://s.ytimg.com/yts/cssbin/www-embed-player-vflx2vAgB.css (238 KB), and
http://www.visahochieu247.com/wp-content/themes/sahifa/style.css (111 KB).
Also, remember that each script is evaluated before making another GET, further slowing the process.
Your images are reasonably sized, but there are 44 of them and each requires a get.
Don't forget that each file takes a separate GET and each GET involves handshaking between server and client to establish transfer methods. That takes time, especially for 73 total GETs.
Client side caching will make subsequent pages load faster, as once loaded the client will not reload the file again. You cannot depend on that since users may have set caching to NIL for their own reasons.
I hope my guess was helpful. If not, please provide more detail.