Correct me if I'm wrong (sure someone will) but flight plan speeds relate to cruise speeds and are transmitted as TAS. There is no such thing as a flight plan speed for the descent. So what's a slow speed? When do you want us to tell you? Having flown three different boeing twins so far in my career, each one has a diffrent econ descent speed. Until there is a dictat that 300kts (for example) is a mandatory speed, then its up to the commanders choice on the day. Subject of course to ATC decreeing otherwise.