Normall convolution
approximate complexity =
out_dimxkernelxconv= 4 x 9 xconv. (conv=in_dimxin_pixels)

Separable convolution
Separable conv = Depth-wise conv + Point-wise conv
approximate complexity =
kenelxconv+out_dimxconv= (9 + 4) xconv. (conv=in_dimxin_pixels(

Animation and videos
(Youtube video) Groups, Depthwise, and Depthwise-Separable Convolution (Neural Networks)