That’s not so simple with time and materials billing. If somethings taking too long to deliver or is being delivered to a low standard, it could easily be explained by having to deal with unplanned problems (which could also be entirely the clients fault), or it could just as easily be a contractor being slow and/or useless. I’ve worked as a contractor for quite a long time, and the number of complete dead weight contractors I run into is astonishing, and so is the number of projects that go way over budget due to poor planning from the clients. The metric you propose to track doesn’t do such a good job of telling the difference between these two things. This approach doesn’t do a terribly great job of that either, but delivering your work under the conditions chosen by your client is pretty much how it always is for contractors/consultants/freelancers. If they take things too far they won’t be able to attract the high quality contractors in any case.