I am working on a tool to measure the speed of work by engineering teams. One visualization shows whether work is speeding up, slowing down, or going at a constant rate. What would you call that?
Management calls the amount of work a team completes during a sprint 'velocity'. They usually say it is increasing or decreasing. Most agile tools already have this measure in them (see JIRA).
In fields outside of computer science and technology in general, the rate of change in velocity or speed is usually called acceleration, or deceleration, as appropriate.