Might be a basic question: is the clock rate (1.7 GHz, etc.) that is mentioned for multi-core processors (e.g., Intel i5, i7) the clock rate per core, or it is the total speed of all cores combined?
Each core runs at the clock rate.
Just doing a fast search did not appear in that recommended linked answer, so I'll mention here. Consider that most of this cores have what is called a turbo mode: Not technically well explained but basically that mode can be triggered, when using one or less cores than the 4 ones,(running a single core app or other situation) so that it can run at higher clock rates. IE, the i7 870's each core rate might be 2.93MHZ, but in turbo mode, a single core app can run at 3.6 GHz. Not all the available cpus support this, but i7 cpus ones can, for example (some i5 do not have turbo, none of the i3 have it.).
It is neither. Think of each core like a car on the highway. If there are four cars on the highway going 50 miles per hour, is that 50 miles per hour "per car"? Of course not. Is it a total speed? Of course not.
The clock speed is the rate at which the processor's clock ticks. It isn't a sum, and it isn't per anything. In particular, it is not the rate at which work is done.