DirOptions.Cacheis the field you have to enable.
DirCacheOptionsallows to cache files based on the available compressions(!) too. So if a client requests a gzip version of a file, then the server will be serve the gzip data directly, without the necessity to encode it on fly. This can improve your site's speed to over 50%. This is done automatically by the framework, you just have to provide available Encodings that you want to support and Iris can handle the rest work.
iris.MatchImagesAssetsis just a regular expression which ignores to compress all pre-compressed images for a second time, e.g.