Q:

How do the execution speeds of snprintf and strncat compare?

I have a program which concatenates some number of strings in a buffer.
I was using strncpy before. After looking into some suggetions on web to use snprintf instead of strncat, I switched to snprintf. However, I have noticed a delay in execution of this part of program(string concatenation) then before. Is this possible, that snprintf can slow down the program exucution speed? If not then I will look for some other factors in my program.
if(iOffset<MAX_BUFF_SIZE && iOffset>0)
{
  ....... 
  iOffset += snprintf(cBuff+iOffset, sizeof(cBuff)-iOffset, "%s", "String1");
  ...... 
}

I repeat the above code snippet like 12 times in order to append strings to cBuff.
Edit : It repeats 12 times after each 1 sec.

A:

There are a couple of suggestions that come to mind:

don't optimize your program too early
when you're ready to optimize, use a tool like a profiler to discover where the real slow downs are

If you're not familiar with profiling, or are really eager to know the timing details you can always include timers around your code.  See references on gettimeofday() or clock() and the appropriate caveats about those codes.  Essentially you can get the time before execution, the time after and compare them.  Comment out your line of code and put in the old code and time it.
With all that said ... that's part of what a profiler does for you.  Figuring out "why" something is slow is sometimes complex because there may be other considerations going on (ie at the hardware level) that you're not aware of.
The execution of a function 12 times is probably trivial compared to the total execution time of the program.

A:

strncat suffers from the Schlemiel the Painter problem. The way you're using snprintf does not (though it's possible to use strncat that way too, and sidestep the problem that way).

