Data storage is one example, a lot of time loading of data from HDD was the n°1 performance issue as it was thousands of time slower than CPUs ; nowadays the fastest SSDs have sequential reads that read at 12GB/s (DDR3-1600 speeds!) which can easily be much faster that whatever non-trivial processing you'll apply to the data, so the bottleneck moved from IO to CPU.

There's two ways to understand that. What I think is the likely intention is the requirements may change, such that pieces that currently don't do much computation start to do a lot. Easiest one to think about is adding encryption. Your http server may have been bottlenecked on disk or network i/o, but your https server may be bottlenecked on cpu; not a great example because the cpu work in TLS is in bits and pieces and will probably share nicely.

The other way is that sometimes a little more load perturbs the system so things take a lot longer. I've had systems that work fine until you hit X% cpu, then it jumps to 90%+, because of say lock contention; sometimes N is 80% so whatevs, but sometimes it's 25%, and then you need to find and fix the bottleneck.

The transition from spinning disks to SATA/SAS flash to NVMe.