Transaction costs on Interledger can potentially be much lower than the costs of the underlying system used for settlement, since Interledger peers extend credit to one another. Depending upon how much credit is extended, settlements may be performed infrequently, or unnecessary if flows net out. So, many Interledger payments may be settled by only a single transaction of the settlement asset. And the fewer settlements that are required, the lower the Interledger transaction costs, since there’s less transaction costs from the settlement system.
As a minor clarification, no bridge asset is required to use Interledger, although that’s one theory of adoption.
Interledger transactions per second (for the most part) is independent of the TPS of the settlement system used between two peers. Since Interledger payment packets are cleared off chain, Interledger TPS is likely orders of magnitude greater than e.g. blockchains which require distributed consensus.
So yes, for all intents and purposes, Interledger TPS is practically unlimited…!