A student ran numerical simulation on a dual$-$core computer Smartcache, $32GB$ RAM$).$ Each simulation involve calculation of $2$GB of data and takes about $3$ hours. To speed up the calculation, he ran two simulations simultaneously. But it turned out that the computer had taken $8$ hours to complete the jobs, i$.$e$.,$ the average time to complete one job is $4$ hours. Explain why the performance is poorer than only running one simulation at a time. $[12$ points$]$