As you can understand, it all depends on your server’s configuration, network latency, version of PHP, etc. I have not made any specific measurements, nor would these be useful, since every server is different. I am simply issuing a warning because some people think that they can be the next Bittrex by installing a plugin on their WordPress. I have made significant effort to keep the number of queries down to a minimum for order placement, with some creative SQLing 🙂
Having said this, it’s a good idea to run the latest PHP and MySQL version possible, if you are interested in performance. PHP 7 is hundreds of times faster than PHP 5.