How can I make my cellular automata quicker - c++

Question

I have a grid of 300x300 cells. I am checking each cell individually which means I am checking 90000 cells atleast every second. Now, I know, one second is pretty fast considering the cpu is going through 300^2 different cells. But this stuff just isn't fast enough. I watched a guy on youtube who got his simulation to update every 20 ms. And he was running 512^2 cells. How is this possible? Optimizations? Is my computer a potato? I'll show my code if anyone wants to see exactly what i'm doing.
for (int y = GY; y >= 0; y--)
for (int x = 0; x < GX; x++) {
cell* cell_pointer = &grid[x][y];
if (grid[x][y].CellT == type::SAND) {
if (grid[x][y+1].CellT == type::AIR) {
if (y+1 > GY-1) {
continue;
}
cell* cell_below = &grid[x][y+1];
cell_below->Color ={ 255,0,0 };
cell_below->CellT = type::SAND;
cell_pointer->Color ={ 0,0,0 };
cell_pointer->CellT = type::AIR;
}
else if (grid[x+1][y+1].CellT == type::AIR) {
cell* cell_below = &grid[x+1][y+1];
cell_below->Color ={ 255,0,0 };
cell_below->CellT = type::SAND;
cell_pointer->Color ={ 0,0,0 };
cell_pointer->CellT = type::AIR;
}
else if (grid[x-1][y+1].CellT == type::AIR) {
cell* cell_below = &grid[x-1][y+1];
cell_below->Color ={ 255,0,0 };
cell_below->CellT = type::SAND;
cell_pointer->Color ={ 0,0,0 };
cell_pointer->CellT = type::AIR;
}
}
}
The code above is my update function. I am updating the cells bottom to top, left to right. It's a sand simulation which is why we're checking the bottom left, bottom right as well as the bottom neighboring cells.
//Square//
SDL_SetRenderDrawColor(renderer, 0, 0, 255, 255);
SDL_RenderClear(renderer);
//////////
int X = floor(mousePos.x/r.w);
int Y = floor(mousePos.y/r.h);
if (left) {
grid[X][Y].CellT = type::SAND;
grid[X][Y].Color ={ 255,0,0 };
}
if (right) {
grid[X][Y].CellT = type::WALL;
grid[X][Y].Color ={ 0,255,0 };
}
for (int y = 0; y < GY; y++)
for (int x = 0; x < GX; x++) {
cell* cell_pointer = &grid[x][y];
SDL_SetRenderDrawColor(renderer, cell_pointer->Color.r, cell_pointer->Color.g, cell_pointer->Color.b, 255);
r.x = cell_pointer->PosG.x;
r.y = cell_pointer->PosG.y;
SDL_RenderFillRect(renderer, &r);
}
SDL_RenderPresent(renderer);
SDL_UpdateWindowSurface(window);
The code above is my draw function. I am using SDL as my graphics API. I am getting the mouse position relative to the grids so that when I click left or right mouse buttons I can draw a sand cell or a wall cell. So I am checking each cell individually top to bottom and left to right and then drawing a square individually where the cell's position is.