time_duration inefficient compared to ptime and gregorian::date

compile this program:
```
#include
#include
#include
#include
#include

using namespace boost::posix_time;
using namespace boost::gregorian;
using namespace std;

int main(int argc, char *argv[])
{
const int64_t count = 1000000;

{
vector v;
v.reserve(count);
for (int64_t i=0; i(finish - start).count() << "s\n";
}

{
vector v;
v.reserve(count);
for (int64_t i=0; i(finish - start).count() << "s\n";
}

{
vector v;
v.reserve(count);
date d(1400,1,1);
for (int64_t i=0; i(finish - start).count() << "s\n";
}

{
vector v;
v.reserve(count);
ptime p(date(1400,1,1));
for (int64_t i=0; i(finish - start).count() << "s\n";
}

return 0;
}
```

program output on my machine:

time_duration sort: 3.60364s
int64_t sort: 0.560309s
date sort: 0.610657s
ptime sort: 1.36204s

the question is, what is so special about time_duration making it so unbelievably inefficient? it is even more inefficient than ptime!