File system much slower than expected

I've noticed a significant slowdown when reusing an ofstream. If I do the following:
```
ofstream output;
output.open( filename, ios_base::trunc | ios_base::out | ios_base::binary );
output.close();
output.clear();
output.open( filename, ios_base::trunc | ios_base::out | ios_base::binary );

```
then writes to the file are 3 to 4 times slower than expected. Removing the last three lines, the speed is fine. Using a different ofstream to open the file is fine. Reusing the same ofstream is slow.