Often 2D arrays are implemented as 1D arrays. Sometimes 2D arrays are implemented by a 1D array of pointers to 1D arrays. The first case obviously has no performance penalty compared to a 1D array, because it is identical to a 1D array. The second case might have a slight performance penalty due to the extra indirection (and additional subtle effects like decreased cache locality).
It's different for each system what kind is used, so without information about what you're using there's really no way to be sure. I'd advise to just test the performance if it's really important to you. And if the performance isn't that important, then don't worry about it.
For C, 2D arrays are 1D arrays with syntactic sugar, so the performance is identical.
