I've been working up a game engine similar to Terraria, mostly as a challenge, and while I've figured out most of it, I can't really seem to wrap my head around how they handle the millions of interactable/harvestable tiles the game has at one time. Creating around 500.000 tiles, that is 1/20th of what's possible in Terraria, in my engine causes the frame-rate to drop from 60 to around 20, even tho I'm still only rendering the tiles in view. Mind you, I'm not doing anything with the tiles, only keeping them in memory.

Update: Code added to show how I do things.

This is part of a class, which handles the tiles and draws them. I'm guessing the culprit is the "foreach" part, which iterates everything, even empty indexes.

...
    public void Draw(SpriteBatch spriteBatch, GameTime gameTime)
    {
        foreach (Tile tile in this.Tiles)
        {
            if (tile != null)
            {
                if (tile.Position.X < -this.Offset.X + 32)
                    continue;
                if (tile.Position.X > -this.Offset.X + 1024 - 48)
                    continue;
                if (tile.Position.Y < -this.Offset.Y + 32)
                    continue;
                if (tile.Position.Y > -this.Offset.Y + 768 - 48)
                    continue;
                tile.Draw(spriteBatch, gameTime);
            }
        }
    }
...

Also here is the Tile.Draw method, which could also do with an update, as each Tile uses four calls to the SpriteBatch.Draw method. This is part of my autotiling system, which means drawing each corner depending on neighboring tiles. texture_* are Rectangles, are set once at level creation, not each update.

...
    public virtual void Draw(SpriteBatch spriteBatch, GameTime gameTime)
    {
        if (this.type == TileType.TileSet)
        {
            spriteBatch.Draw(this.texture, this.realm.Offset + this.Position, texture_tl, this.BlendColor);
            spriteBatch.Draw(this.texture, this.realm.Offset + this.Position + new Vector2(8, 0), texture_tr, this.BlendColor);
            spriteBatch.Draw(this.texture, this.realm.Offset + this.Position + new Vector2(0, 8), texture_bl, this.BlendColor);
            spriteBatch.Draw(this.texture, this.realm.Offset + this.Position + new Vector2(8, 8), texture_br, this.BlendColor);
        }
    }
...

Any critique or suggestions to my code is welcome.

Update: Solution added.

Here's the final Level.Draw method. The Level.TileAt method simply checks the inputted values, to avoid OutOfRange exceptions.

...
    public void Draw(SpriteBatch spriteBatch, GameTime gameTime)
    {
        Int32 startx = (Int32)Math.Floor((-this.Offset.X - 32) / 16);
        Int32 endx = (Int32)Math.Ceiling((-this.Offset.X + 1024 + 32) / 16);
        Int32 starty = (Int32)Math.Floor((-this.Offset.Y - 32) / 16);
        Int32 endy = (Int32)Math.Ceiling((-this.Offset.Y + 768 + 32) / 16);

        for (Int32 x = startx; x < endx; x += 1)
        {
            for (Int32 y = starty; y < endy; y += 1)
            {
                Tile tile = this.TileAt(x, y);
                if (tile != null)
                    tile.Draw(spriteBatch, gameTime);

            }
        }
    }
...