What operating system do you use? If MS Windows, Windows is not a Real Time Operating System. Every time a lot of threads work in your computer at the same time. And your CPU has no more than 8 cores (for Intel Core i7).
So, OS performs time quantization for all threads on your PC. As a result, time to complete operation will be slightly different in any case.
One of possible solutions is to repeat measure process many times, and get average.
ippGetCpuCLocks functions returns readings of RTC - real-time counter. It's is analog of wall clock.
It cannot be slower or faster, it just ticks)). The difference between two readings shows the amount of time spent between "before" and "after".
How fast you get from "before" to "after" depends on what Roman is writing about.