I think you are saying 2 different things, in your definition of constructive feedback 1.) The feedback should be nice/not brash/not toxic. 2.) The work needs

I think you are saying 2 different things, in your definition of constructive feedback 1.) The feedback should be nice/not brash/not toxic. 2.) The work needs to be done by the person to give more detailed feedback. While 2.) is a not a reasonable expectation it is pretty simple to achieve 1.), by keeping emotions out of your feedback. For example I would just go with "The webservice is slow, could you take a look? It is affecting xyz".