Time Consuming by cvEigenDecomposite

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Dears ,
I am trying to implement face recognition using pca ,
when I user the function "cvEigenDecomposite" to project test img in face space , I measured the time consuming by this function in different situations

When Training = 800 img  &  # eigen values = 799   ---------> time = 181 sec  
When Training = 1000 img  &  # eigen values = 999   ---------> time =  72  sec    !!!!!!!!!!!!!

is this logic ???????????
why 1000 takes smaller time ??????????

for (i = 0; i < iTestFaces; i++){ 
    subFuncTime_Decomposite = GetTickCount(); 
    cvEigenDecomposite(pFaceImageArr[i], iEigens, pEigenVectArr, 0, 0, pAvgTrainImg, pProjectedTestFace); 
    subFuncTime_Decomposite = GetTickCount() - subFuncTime_Decomposite; 

    subFuncTime_iNearestPerson = GetTickCount(); 
    iNearestPerson = FindNearestNeighbor(pProjectedTestFace, iNearestDistance,MyID ,ResultFileObj, ResultExcelObj); 
    subFuncTime_iNearestPerson = GetTickCount() - subFuncTime_iNearestPerson;
}