GDI+ - refresh too slow

Hi,

I have an application (managed c++) that works on two dimensional array of integers. Its size is user-defined, but let's assume it's 500x500. Each value represents different object - assume there are about 11 distinct values (0 - empty field and 1..10 - kinds of objects). I want to visualize it. So far I set up Graphics (g = this->CreateGraphics()) Pen (p) and SolidBrush (b) objects. Then I put loop
                         for(int i = 0; i < lattice->board->sizeX(); i++) {
                               for(int j = 0; j < lattice->board->sizeY(); j++) {
                                     if(lattice->board->getPlankton(i, j) != nullptr) {
                                          b->Color = lattice->board->getPlankton(i, j)->printColor();
                                          g->FillRectangle(b, offset_x + pix_size*i, offset_y + pix_size*j, pix_size, pix_size);
                                     }
                               }
                         }
Everything is neat and nice, as desired.

The problem is that this is simulation of integers in this array...when theirs quantity (!= 0) gets higher, refreshing image gets really slow. I can see how it goes column-by-column...and it's unacceptable for me.
I've already tried to set up double buffering with
                         this->SetStyle(static_cast<ControlStyles>(ControlStyles::DoubleBuffer |
                              ControlStyles::UserPaint |
                              ControlStyles::AllPaintingInWmPaint),
                              true);
                         this->UpdateStyles();
but this doesn't help me.

The questions are:
(1) what can I do to improve my performance?
or
(2) I do not insist on drawing it like that, I just need to have this array displayed in any way. Maybe there is better way to accompish that?

Thanks,
Grzegorz