While slow in absolute depends on lots of factors, the relative slowness of things doesn't so much. Whatever the app or the device, accessing a register is always going to be faster than random places in RAM, which is always going to be faster than fetching something on disk and even moreso if we talk about fetching stuff over the network. No matter how hardware progresses, latency hierarchy is doomed to stay.
That doesn't mean it's the only factor of slowness, and that async/await solves all issues, but it's a tool that helps, a lot, to fight against very common sources of performance bugs (like how the borrow checker is useful when it protects against the nastiest class of memory vulnerabilities, even if it cannot solve all security issues).
Because the situation where “my program is stupidly waiting for some IO even though I don't even need the result right now and I could do something in the meantime” is something that happens a lot.
> Whatever the app or the device, accessing a register is always going to be faster than random places in RAM, which is always going to be faster than fetching something on disk and even moreso if we talk about fetching stuff over the network.
The network is special: the time it takes to fetch something over the network can be arbitrarily large, or even infinite (this can also apply to disk when running over networked filesystems), while for registers/RAM/disk (as long as it's a local disk which is not failing) the time it takes is bounded. That's the reason why async/await is so popular when dealing with the network.
That doesn't mean it's the only factor of slowness, and that async/await solves all issues, but it's a tool that helps, a lot, to fight against very common sources of performance bugs (like how the borrow checker is useful when it protects against the nastiest class of memory vulnerabilities, even if it cannot solve all security issues).
Because the situation where “my program is stupidly waiting for some IO even though I don't even need the result right now and I could do something in the meantime” is something that happens a lot.