The faster card will not slow down, they will both run at whatever they are set to.
The cards are given equal work though, the frame will not be drawn until the slower card is done. Thus the performance is always the same as crossfire of the slowest card.
However, in games which do not utilize CF, if the faster card is the primary card it will continue to work at stock, not equal to a single of the lesser card.
SLI on the other hand will match clock settings.