[pontus]

Yes, that's right but that has to do with entanglement in QM and is not specific to MWI. In classical mechanics, a system of n particles is specified by 3n different functions of time - the three coordinates for each of the n particles. The complexity in terms of e.g. memory then scales linearly with the number of particles.

In QM by contrast we have entanglement, which essentially means that we can't describe one particle separately from all the other particles (if we could, then QM would be just as "easy" to solve as classical mechanics). Instead of 3n functions of time, we instead have a single function of 3n variables (plus time). The complexity of these functions does not scale linearly with n (imagine e.g. a Fourier series in one variable vs one for two variables)

So, you're right that QM is an exponentially harder problem to solve compared to classical mechanics, but this is because of entanglement and has nothing to do with Copenhagen vs MWI.