SSD is pretty fast; but my app is actually trying to do more than 100_000 read-modify-write cycles per second and that still requires careful thought about the database and schema we're using.

CPU and RAM are pretty fast. I do a live-coding interview question and I ask people do to a naive implementation first, then later I ask about possible optimizations. A third to a half of candidates want to do fewer RAM accesses and oh by is that the wrong avenue for this problem - especially when they just wrote their solution in Python and you could get a 10x-20x speedup by rewrite in C/C++/Go/Rust/etc.

Network is IO too. Network is pretty fast, datacenter-to-datacenter, but end users can still have their experience improved with better encoding and protocol; and outbound bandwidth bills can be improved by that too.