There is a really interesting note here: http://en.cppreference.com/w/cpp/chrono/c/clock
"Only the difference between two values returned by different calls to std::clock is meaningful, as the beginning of the std::clock era does not have to coincide with the start of the program. std::clock time may advance faster or slower than the wall clock, depending on the execution resources given to the program by the operating system. For example, if the CPU is shared by other processes, std::clock time may advance slower than wall clock. On the other hand, if the current process is multithreaded and more than one execution core is available, std::clock time may advance faster than wall clock."
Why does the clock speed up with multithreading? I'm checking the performance of a C++ program with threading vs without it and I'm noticing that the times are similar for threading (not better) but feel faster (like saying 8 seconds in 3 seconds of runtime).
If more than one core is available, and you are running multiple threads, then potentially multiple threads are executing at the same time on different cores. Since clock() measures processor time, it may advance faster than wallclock time, because multiple threads are advancing it simultaneously.
Just as the example given in the documentation - it shows two threads created, and the clock() value reported is almost double the wallclock time reported.
来源：https://stackoverflow.com/questions/35592502/clock-function-in-c-with-threads