Why the app will run slower when it occupied more memory?

[tjuzhangrui]

When the app's memory usage is about 30M bytes, it runs fast.
But if it allocates a continuous big block of memory such as 100M bytes, while the cpu's load is constant and the physical memroy is enough. It will run much slower.

for example

//MyDoc.h
class MyDoc : public CDocument
{
      //just allocate a big blcok of memory,but it won't be used
      //as a result , the app will run much slower
     double m_d[1024*1024*25];
}

What's the reason?

Thanks!