Scalability
How much transaction volume a network can handle without fees or delays exploding.
Scalability is usually quoted in transactions per second, though finality time and cost per transaction matter just as much in practice. Bitcoin settles a handful per second at base layer; high-throughput chains claim thousands.
Raising throughput on the base layer generally means larger blocks and heavier nodes, which prices out independent operators, so rollups and sharding are the preferred routes.