In my experience, it does not matter much for in terms of speed. The reason is that the size of an optimization really does not correlate with the time it takes to solve the model. There are even examples, where an equivalent formulation with significantly less variables and constraints is significantly harder to solve (see e.g. here.
Therefore, I always add those aggregate variables since they make the modeling cleaner. Then I give it to the solver and see whether it solves according to the performance that I need. If yes, great. If not, they you may want to have another look at the model formulation (and other details like computer hardware, solver parameters etc.) to see whether there are any improvements to be had.