A page fault (something that isn't in cache, so the CPU has to send off to get it for ram, a process that takes hundreds to thousands of cpu cycles) leaves the cpu 100% utilized, but doing nothing.
To monitoring software there is no difference between constant 1,000 cycle page faults (so operating maybe 10 cycles per 1000) and doing short integer/floating point math that does an operation every cycle.
Both show up as 100% usage, but the page fault version is only actually doing something 1% of the time, while the short math version is doing something more like 90-95% of the time (getting stuff from L2/L3 (especially L3) takes time too).
An even nastier program would do both integer and floating point math at the same time, most modern CPUs do those two types of math in different parts and hence can do them at the same time if neither process requires any of the same data/registers.