It takes so long because the GIMP developers have a long-standing tradition of telling people with requests to go jump off a cliff if the feature being requested isn't directly useful to the developers. They have a fairly typical failing in assuming that because the source is available, anyone asking for features has the ability and the time to implement those features themselves, when often that is not the case.
So I'm thinking one way to reduce the barrier for entry into participation is to have the bulk of the application's features in a higher level language (rather than C), with the core in a high performance language. Unfortunately this is not something I'd have the time to pursue.
Of course, if you don't care whether or not anyone uses your software, telling people with questions and requests to jump off a cliff is entirely reasonable.
