most c compilers generate calls to _ftol or something for every float to int conversion. putting a reduced floating point conformance switch (like fp:fast) might help - IF you understand AND accept the other effects of this switch. other than that, put the thing in a tight assembly or sse intrinsic loop, IF you are ok AND understand the different rounding behavior.
for large loops like your example you should write a function that sets up floating point control words once and then does the bulk rounding with only fistp instructions and then resets the control word - IF you are ok with an x86 only code path, but at least you will not change the rounding.
read up on the fld and fistp fpu instructions and the fpu control word.
