Smart Contracts are fast and performant to allow developers to build anything. The design goal is that developers can build consumer-facing experiences that are as fast as they would expect from centralized servers. Dapp developers do not need to choose between "smart contracts" and "fast." In short, dapp users experience is so fast that users should not notice if their web experience is running on a blockchain or centralized provider.
From a blockchain POV, Internet Computer Performance performance tests shows the IC latency at 200 milliseconds for query calls (reads) and 2 seconds for update calls (writes). As of December 1, 2021, The Internet Computer can handle 250,000 queries per second and 11,500 update calls per second.