I doubt reaper adds much at all. All reaper does is change the numbers used in calculations. Except for the addition of Reapers themselves, it does not increase the amount of calculations per player. The thing most likely to consume the lion’s share of cycles is intersection testing. Did my arrow hit the mob or not? Which critters got hit by my energy burst? Did my beautifully detailed spiky dinosaur tail just run your low HP wizard through?
The more mobs there are, the more players there are, the more detailed your model is, and the more ways they can collide with each other, the hotter the CPU is going to run. The actual numbers that are plugged into formulae make little to no difference. Reaper trees simply are not the problem (unless they are re-summed on every single interaction, which they no longer are -right SSG?). Reaper mobs on the other hand, champs, new spells, and the latest fiddly bits that introduce NEW calculations per player, combined with many new players will soak up cycles.
The solutions can include making players angry by making the world less detailed to reduce intersection calculations, to the expensive by paying for better hardware. Contrary to popular opinion, better hardware is often a reasonable choice when faced with increased load, but it may be difficult in a fixed data center, or linearly expensive in AWS for example. It is a lot easier than dumbing the game down though.
