Like any big computation, it's lots of simple stuff smashed together, so if the simple stuff is slightly slow it can cost a lot. Nonlinear mixed effects model f

Like any big computation, it's lots of simple stuff smashed together, so if the simple stuff is slightly slow it can cost a lot. Nonlinear mixed effects model fitting with a few thousand patients (like in a later stage clinical trial) can take 2 weeks to one month to run. Being slow at this stage of the computation can delay clinical trials. Because of the extreme cost of these trials, people very much care if it's too slow.