pthread multithreading in Mac OS vs windows multithreaing - c++

Question

I've developed a multi platform program (using the FLTK toolkit) and implement multithreading to do background intensive tasks.
I have followed the FLTK tutorials/examples on multithreading which involve using pthread on Mac, ie the function pthread_create and windows threading on windows ie _beginthread
What I have noticed is that the performance is much higher on Windows ie 3 to 4 times faster in these background threads (in the time to execute them).
Why might this be? Is it the threading libraries I'm using? Surely there shouldn't be such a difference? Or could it be the runtime libraries underneath it all?
Here are my machine details
Mac:
Intel(R) Core(TM) i7-3820QM CPU # 2.70GHz
16 GB DDR3 1600 MHz
Model MacBookPro9,1
OS: Mac OSX 10.8.5
Windows:
Intel(R) Core(TM) i7-3520M CPU # 2.90GHz
16 GB DDR3 1600 MHz
Model: Dell Latitude E5530
OS: Windows 7 Service Pack 1
EDIT
To just do a basic speed comparison I compiled this on both machines running from the command line
int main(int agrc, char **argv)
{
time_t t = time(NULL);
tm* tt=localtime(&t);
std::stringstream s;
s<< std::setfill('0')<<std::setw(2)<<tt->tm_mday<<"/"<<std::setw(2)<<tt->tm_mon+1<<"/"<< std::setw(4)<<tt->tm_year+1900<<" "<< std::setw(2)<<tt->tm_hour<<":"<<std::setw(2)<<tt->tm_min<<":"<<std::setw(2)<<tt->tm_sec;
std::cout<<"1: "<<s.str()<<std::endl;
double sum=0;
for (int i=0;i<100000000;i++){
double ii=i*0.123456789;
sum=sum+sin(ii)*cos(ii);
}
t = time(NULL);
tt=localtime(&t);
s.str("");
s<< std::setfill('0')<<std::setw(2)<<tt->tm_mday<<"/"<<std::setw(2)<<tt->tm_mon+1<<"/"<< std::setw(4)<<tt->tm_year+1900<<" "<< std::setw(2)<<tt->tm_hour<<":"<<std::setw(2)<<tt->tm_min<<":"<<std::setw(2)<<tt->tm_sec;
std::cout<<"2: "<<s.str()<<std::endl;
}
Windows takes less than a second. Mac takes 4/5 seconds. Any ideas?
On Mac I'm compiling with g++, with visual studio 2013 on windows.
SECOND EDIT
if I change the line
std::cout<<"2: "<<s.str()<<std::endl;
to
std::cout<<"2: "<<s.str()<<" "<<sum<<std::endl;
Then all of a sudden Windows takes a little bit longer...
This makes me think that the whole thing might be compiler optimisation. So the question would be is g++ (4.2 is the version I have) worse at optimisation or do I need to provide additional flags?
THIRD(!) AND FINAL EDIT
I can report that I achieve comparable performance by ensuring g++ optimisation flags -O were provided at compile time. One of those annoying things that happens so often
A: Im tearing my hair out on problem x
B: Are you sure you're not doing y?
A: That works, why is this information not plastered all over the place and in every tutorial on problem x on the web?
B: Did you read the manual?
A: No, if I completely read the manual for every single bit of code/program I used I would never actually get round to doing anything...
Meh.