$Version
{m,n,p}={2000,2000,2000};
runs=50;
AA=RandomReal[{0,1},{m,p}];
BB=RandomReal[{0,1},{p,n}];
CC=RandomReal[{0,1},{m,n}];
ListPlot[Table[ Timing[CC=AA.BB;][[1]],{runs}],
PlotRange->All]

$Version
{m,n,p}={1000,1000,1000};
runs=50;
AA=RandomReal[{0,1},{m,p}];
BB=RandomReal[{0,1},{p,n}];
CC=RandomReal[{0,1},{m,n}];
ListPlot[Table[ Timing[CC=AA.BB;][[1]],{runs}],
PlotRange->All]