High level, Ethereum is intended as "computer", and IPFS is intended as "storage". They complement eachother well.
Beyond that, it depends on what you mean by "computations".
Our example is setting up decentralized email. What computation is needed to send and receive email?
1. You may mean the computation needed to encrypt and decrypt the emails. That would happen on your computer locally.
2. You may mean the computation to send and receive emails. I'm not sure how/where this would happen in this example. It might be on the blockchain somehow.
