The most valuable resource on the chain is gas, because the gas of each block is capped, and the average block-out time is fixed, so there is actually a limit to the number of computational steps that can be done per unit of time:
TPS = (Max_Gas_Per_Block/Gas_Per_TXN)/Block-out_Time
Obviously, we need to reduce gas per txn.
The direct way is to package multi txn into one, and that's the core idea of rollup.