Ensure our website is ready for temporary surge of new users

If we get a sudden surge of new users, our website will have to serve many more hits, e.g. to the UDFs and IDFs. We should benchmark how many concurrent requests it can currently handle, and if that’s not enough, adjust the webserver config & resources.

Relevant stress testing tools: ab (apache2-utils package), siege, tsung.

Parent Task: #12406

Related issues

Related to #16091
Blocked by #14588 (closed)

Original created by @intrigeri on 12408 (Redmine)

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