The RAN has had standard routing speeds for decades. All ship ranges are quoted on that basis, whether they are likely to actually participate in warfighting operations with the fleet or not. That is because while they might not be involved in actual warfighting operations with, say, a DDG, there will be times when they are in company, conducting OOW manouevres, towexs, distexs or just on passage together. And operations do not just include high end warfighting; they include HADR, sovereignty enforcement, SAR ops, showing the flag, nation building, all sorts of activities during which, in the past, patrol boats and MFUs have operated together and it can be expected OPVs and MFUs will operate together in the future. Further, it has been specifically stated that the Arafuras will have the capability of contributing to the tactical picture which requires them to be able to keep up with the tactical picture.
A warship is a versatile capability; they will all do a wide variety of things over their lives and one doesn't limit them unnecessarily.