Optimistic Rollup explained

The ‘scalability trilemma’, in simplistic terms, argues that it is difficult to simultaneously satisfy the following three properties — decentralization, security, scalability — where decentralization refers to users validating the chain with limited resources, scalability refers to being able to process a large number of transactions, and security refers to being able to withstand attacks. Trade-offs must be made, and to date, blockchains like Ethereum and Bitcoin have opted to prioritize decentralization and security over scalability.

Achieving high scalability — measured in transactions processed per second — is ultimately necessary if blockchain networks wish to viably compete against legacy payment rails and financial services, providing an indistinguishable user experience from existing ‘Web 2.0’ applications.