Re: C++ is Slow?

I'm constantly confronted with the following two techniques, which I
believe often produce less readable code, but I am told are faster
therefore better. Can anyone help me out with counter examples and
arguments?

1. The STL is slow.

More specifically vector. The argument goes like this:

"Multidimensional arrays should be allocated as large contiguous
blocks. This is so that when you are accessing the array and reach the
end of a row, the next row will already be in the cache. You also
don't need to spend time navigating pointers when accessing the array.
So a 2 dimensional array of size 100x100 should be created like this:

const int xdim=100;
const int ydim=100;

int *myarray = malloc(xdim*ydim*sizeof(int));

and accessed like this:

myarray[xdim*ypos+xpos] = avalue;

Is this argument reasonable? (Sounds reasonable to me, though the
small tests I've performed don't usually show any significant
difference).

To me this syntax looks horrible, am I wrong? Is vector the wrong
container to use? (My usual solution would be a vector<vector<int> >).
Would using a valarray help?

2. iostream is slow.

I've encountered this is work recently. I'd not considered it before,
I like the syntax and don't do so much IO generally... I'm just now
starting to process terabytes of data, so it'll become an issue. Is
iostream slow? specifically I encountered the following example
googling around. The stdio version runs in around 1second, the
iostream version takes 8seconds. Is this just down to a poor iostream
implementation? (gcc 4 on OS X).

Or are there reasons why iostream is
fundamentally slower for certain operations?

Are there things I should
be keeping in mind to speed up io?

// stdio version
#include <cstdio>
using namespace std;
const int NULA = 0;
int main (void) {
 for( int i = 0; i < 100000000; ++i )
   printf( "a" );
       return NULA;
}

//cout version
#include <iostream>
using namespace std;
const int NULA = 0;
int main (void) {
 std::ios_base::sync_with_stdio(false);

 for( int i = 0; i < 100000000; ++i )
   cout << "a" ;
       return NULA;
}