DDS vs JPG,PNG

 https://stackoverflow.com/questions/1460579/i-thought-dds-files-were-meant-to-be-quick-to-load

 You need to distinguish between the format that your files are stored in on disk and the format that the textures ultimately use in video memory. DXT compressed textures offer a good balance between memory usage and quality in video memory but other compression techniques like PNG or Jpeg compression generally result in smaller files and/or better quality on disk.

DDS files have the advantage that they support DXT formats directly and are laid out on disk in the same way that DirectX expects the data to be laid out in memory so there is minimal CPU time required after they are loaded to convert them into a format the hardware can use. They also support pre-generated mipmap chains which formats like PNG do not support. Compressing an image to DXT formats is a fairly time consuming process so you generally want to avoid doing it on load if possible.

A DDS file with pre-generated mipmaps that is the same size as and uses the same format as the video memory texture you plan to create from it will use the least CPU time of any standard format. You need to make sure you tell D3DX not to perform any scaling, filtering, format conversion or mipmap generation to guarantee that though. D3DXCreateTextureFromFileEx allows you to specify flags that prevent any internal conversions happening (D3DX_DEFAULT_NONPOW2 for image width and height if your hardware supports non power of two textures, D3DFMT_FROM_FILE to prevent mipmap generation or format conversion, D3DX_FILTER_NONE to prevent any filtering or scaling).

CPU time is only half the story though. These days CPUs are pretty fast and hard drives are relatively slow so sometimes your total load time can be shorter if you load a smaller compressed file format like PNG or JPG and then do lots of CPU work to convert it than if you load a larger file like a DDS and just do a memcpy into video memory. A common approach that gives good results is to zip DDS files and decompress them for fast loading from disk and minimal CPU cost for format conversion.

Compression formats like PNG and JPG will compress some images more effectively than others. DDS is a fixed compression ratio - a given image resolution and format will always compress to the same size (this is why it is more suitable for decompression in hardware). If you're using simple non-representative images for testing (e.g. a uniform colour or simple pattern) then your PNG file is likely to be very small and so will load from disk faster than a typical game image would.