A "slow CW frequency" is any frequency on which slow CW (however it's defined as "slow") is being used for the exchange of call signs, names, signal reports, weather, and other such data.
There are no official frequencies mandating code speeds no greater than 5WPM or any other speed; however if a sender is going at 10 WPM but can only receive 5 WPM, he should slow down. I used to send quite fast when I was first licensed as a Novice in Las Vegas. One of my first QSL cards was from a now-SK in Seattle who complimented my fist but cautioned me never to send faster than I could receive. Good advice.