Scalability describes how well a blockchain can accommodate more users, transactions or data without unacceptable increases in delays, costs or operating requirements. It is broader than a single transactions-per-second figure because growth can stress several parts of the system.
Finding the bottleneck
Execution, data storage, network bandwidth and consensus can each limit capacity. Increasing a block's allowance may admit more work, but nodes must still process and propagate it in time. Requiring much stronger hardware can reduce who is able to independently verify the chain.
Different ways to expand
Improvements can include more efficient software, better data handling or processing activity on additional layers. Rollups execute transactions outside the base layer and use settlement and data mechanisms on that layer. Their capacity and security still depend on the particular design, including proof systems, data availability and operator controls.
Measuring useful growth
A meaningful comparison considers sustained throughput, time to finality, fees under load and the cost of running a node. A short benchmark using simple transfers does not establish performance for complex applications. Low observed activity also does not prove low capacity. Scalability claims are most useful when workload, test conditions and security assumptions are stated.