Most defi is on Ethereum, which is making large scaling improvements. Zkrollups right now do several thousand tx/sec without security compromises, and data sharding will expand that by 23X in about a year. This is still probably a low volume ceiling by some definitions, but it's at least in the ballpark of major systems like the VISA network and NASDAQ trades.

Further down the road, more efficient data witnesses give another 10X. Also, the power of individual nodes affects both the number of shards and the capacity of individual shards, so future scaling is the square of whatever Moore's Law has left to give us.