When you draw on a layer, all the strokes that you create with the Brush tool are vector elements. There are two kinds of brushes:
Plain Brushes: By default, the Brush tool creates vector strokes filled with either a colour or a gradient. Vector strokes use a very small amount of memory and can be used rapidly because they do not contain any pixel information, only mathematical functions.
Texture Brushes: This type of brush also produces a vector contour for its strokes, but is filled with a bitmap texture. These textures allows you to produce drawings that have natural looking brush strokes that resemble lines drawn using a crayon or airbrush.
Because you are using a bitmap image mapped inside vector strokes, texture brushes use much more memory and processing time than brush strokes filled with colour. Therefore if you want to make your file size lighter and drawing speed faster, use only regular brushes to draw scenes. If you choose to use texture brushes, is it important to use a reasonable size bitmap for the texture brush.
NOTE When you use a bitmap layer instead of a vector layer, individual vector strokes are not created. Instead, individual pixels are laid down, recording information about the RGBA of each pixel. Because of this, when working with lots of shading and lots of different textures or colours, the bitmap layer is actually more efficient than the texture brush on the vector layer—see About Drawing.