Not to bring everything back to buzzwords, but that actually seems like an interesting application of blockchain tech, letting people rent out their otherwise idle desktops to process and serve web services.
1. You need to be able to verify that the other party is honest
2. When the data is intensive, you need to make sure that the computation is opaque to the computer. This is still an active research area, I believe. One of the keywords I associate with it is homomorphic encryption. Alternatively, consensus or verification can be done, with other zero-knowledge proofs or something similar perhaps.