Question about DGEMM

I am using DGEMM from MKL to do multiplication between matrix and vectors.

I found when I test in simple program, just calling DGEMM 200000 times to compute 256*256 matrix times 256*1 vector, it takes only about 7 seconds (nthreads=8).

I my real poisson solver, which need this multiplication 200000 times, still 256*256 matrix times 256*1 vector, it takes 2 min, which is much much slower than in simple test. 

Could anyone suggest any reason about this low performance? My poisson solver is openmp code.

Thanks in advance!

Sincerely,

Xuan