after the message, jira becomes so slow, you can't even view an issue.
Sorry to hear that you are seeing this communication timeout error when trying to assign a user in Jira.
Could you please let us know what specific version of Jira you are using here? I recall a similar type of error that was a bug in some older 7.0-7.2 versions of Jira, but I would like to try to confirm which bug this was specifically.
In addition to that, I would be interested to see if the $JIRAHOME/log/atlassian-jira.log file is recording any kind of WARN or ERROR stacktrace when this error message is created. It is possible this could give us additional context clues here as to the actual cause of this problem.