We are experiencing that problem while "Convert URLs to Page Titles" feature is enabled in Admin > Messages is enabled.
If we paste only one(1) Url and send the post, page is waiting a some reply then we see the loading icon top right corner and later sending the post. It is too slow if we compare with sending regular posts. But two (2) different link is impossible to send.
I disabled the Convert URLs to Page Titles feature from admin panel and all messages which including links two or more, sending immediately and fast.
Can i need a set/change something in server side ?
Php version: Php 7.0.2 / Nginx 1.9.9 / php-fpm