Of course Matlab gets slow during the execution if it has a lot do to. This may be the nature of the problem, e.g. if huge data sizes have to be processedm or a problem of the programming, e.g. for iteratively growing arrays, creation of a large number of graphic objects (instead of updateing the XData, YData, ZData of existing objects).
Matlab's graphics output is not fast. If the job requires e.g. the rendering of complex 3D objects, other programming languages migth be faster. Without knowing any details about the actual problem, it is impossible to draw a reliable conclusion.