There is a major issue to bear in mind when compressing images: some of them are already compressed. However, all image compression formats are not equal so here is a breakdown of the most popular types and suggestions on how to handle them:
- JPG - uses lossy image domain compression, not to be confused with HTTP compression as used by IISxpress. This type of compression makes the image data appear to IISxpress to be quite random and therefore difficult to compression. Usually you are best to avoid compressing images in this format.
- GIF - uses an older lossless compression format similar to HTTP compression. However since this is an old format it is not as efficient as it could be. Therefore it is possible to compress these file types by up to 20%.
- PNG - uses a more modern (than GIF) lossless compression format almost identical to HTTP compression. You should generally not compress files of this type.
- BMP - this file format doesn't use compression at all (usually). You should therefore always elect to compress files of this type. This file format isn't normally used for web images, apart from ICO files (see below).
- ICO - a relative of the BMP file format (although usually containing much smaller images). Used by the favicon.ico file which is requested by some browsers. It is usually a good idea to compress files of this type.
Regarding your question about request/response volumes, we load IISxpress with up to 1TB of compression data before we release to make sure it can handle high volumes. We mix the size and type of the response data to be representative of real world traffic. For an HTML file of approx 2.5KB in size IISxpress will compress it approximately 260 times a second, if it is found in the cache then this rises to about 400 times a second. For larger files of 22KB the numbers are around 170 responses a second and 320 responses a second respectively.
Your example of 2 million hits a day works out at about 23 requests per second, based on the numbers above this seems easily achievable. Of course before you deploy IISxpress or any other product on a high volume site you should probably do your own performance and load testing.