Hello Peiyouliao,
I am sorry for the delay of this reply. Have you set cronjobs for easysocial email notification? I've checked your site and it seems like the following option is enabled, backend > easysocial > settings > general > email transportation behavior > Process email on page. When this option is enabled, easysocial will process all emails notification on your site every time your easysocial site and page is being access, hence slowing the site down.
We recommend you to use cronjob and turn off the setting that I mention above. You can refer to our documentation here,
http://stackideas.com/docs/easysocial/administrators/cronjobs for more info on how to setup cronjobs.