What does it matter whether it's efficient or not? It is part of the spec. Compression is in the spec as well.
The DPX format stores image data by pixels, separating each pixel into its component values (also called samples in other formats) and storing each value in a separate element (also called a color plane). Image data components may be stored in up to eight elements total.
For example, an 8-bit gray-scale pixel contains only one component and is therefore stored as one component value only in one element. A 32-bit RGBA pixel contains four components that may be stored across four elements as four 8-bit component values or may be stored in a single element as one 32-bit component value.
As you can see, the DPX format is quite flexible in allowing you to store your image data in any way that makes reading and writing the data the most efficient for your computer hardware.
All components must be the same size across all elements. Valid component sizes are 1-, 8-, 10-, 12-, and 16-bit integers and 32- and 64-bit reals (IEEE floating-point). All components must be stored as words using 32-bit boundaries.
Descriptor specifies the type of component stored by the element and its pixel-packing order. There are 256 possible values to this field; the following are defined:0
User-defined
1
Red
2
Green
3
Blue
4
Alpha
6
Luminance
7
Chrominance
8
Depth
9
Composite video
50
RGB
51
RGBA
52
ABGR
100
CbYCrY
101
CbYaCrYa
102
CbYCr
103
CbYCra
150
User-defined 2-component element
151
User-defined 3-component element
152
User-defined 4-component element
153
User-defined 5-component element
154
User-defined 6-component element
155
User-defined 7-component element
156
User-defined 8-component element
http://www.fileformat.info/format/dpx/egff.htm