I am a patient man, but I don’t actually recall this taking too long. It is quite simple to make r = g = b, the only tricky part is determining which slider in which channel to adjust to achieve natural looking colours. There was some trial and error involved in that.
Perhaps it is more to do with ones comfort level using the channel mixer. It used to be completely foreign to me, but I’m beyond the beginner stage now and its not as scary.