Hi Andy. Royal icing is technically a meringue,  a very heavy one! The egg protein is there to hold the air that is incorporated through the beating process (as in a light meringue). It shouldn’t take that long to reach the right consistency so I think something is amiss. The most likely cause is that grease, oil, fat or a trace of yolk has contaminated the mixture. To avoid this wash the bowl in hot soapy water and rinse well. Allow it to dry without wiping it. Make sure your scales or measuring device are spotless (or weigh onto clean paper). You can wipe the bowl with a cut lemon if you want to be extra careful. Make sure only egg white goes in.  Proportions are 5.5 times icing sugar to whites. That’s 5.5 ozs per medium egg white (approx). If it’s not stiff enough it’s much harder to get the air in. Add the sugar gradually to the whites until all incorporated then beat on high till you get ‘peaks’. That is that the spikes you get when you stick your finger in don’t flop over! You can stir in a teaspoon of glycerine at the end to make the icing softer if you like. Another tip. Add blue colour to make it look white (we used to use those ‘blue bags for washing!) otherwise it ends up looking rather yellowish.  It should set off overnight BTW.   

My Mum used to put gelatin in the icing and it was great. Sort of soft and crunchy at the same time.

Anyway: As far as bread goes, I mention it just ’cause I know that the sciency things happening when you knead bread are pretty complex, and aren’t just a case of the bits being smashed together into submission!

I’d expect royal icing, given all the beating, to be a case of getting little tiny bubbles of air into it to lend it that crumbly, melty character (otherwise it would just be hard and glossy) and keeping it going until the egg starts to congeal to keep the bubbles in? (If this is the case, then things like ambient temperature, the kind of mixer you’re using and the quality of the egg will all have an effect on how long it takes…)