rfndmw,
While calculating max frequency for a circuit, you have to consider the path which requires the largest timeperiod. And these timing paths can be any of the below:
1) Path from input pin to D pin of a Flop
2) Path from clock pin of a flop to D pin of next flop
3) Path from clock pin of a flop to a primary output
4) Path from input to output (only combo)
So while calculating frequency, you have to analyze each of these paths and consider the path that requires largest timeperiod