This is based on what we call "time efficiency". For a single job, time
efficiency is the ratio of total duration to interval. Ideally, this is
close to but smaller than 1.

We use exponential smoothing (EMA) to look at the last 10 jobs and their time
efficiency. With EMA, we tolerate a spiking job better than by just
looking at the most recent job.

We also define upper and lower bounds for the batch size.