If you waiting for it, then yeah you might as well be actively doing it.
Five minutes automated is five minutes available for another part of the game.
Though 5min for 1 items is kinda unreasonable for a building stone. I wholeheartedly agree with unreasonable time, but cannot define [let alone agree on] an "unreasonable" time.
But RC furnaces feel about right for that material- given the amount needed later on and the availability of the iron needed.
Speed buffs via an in-game/mod mechanic can actually be really cool, encouraging further design and ingenuity to use them effectively.
-old IC2's overclockers were pretty neat; how far should/can you push your powergrid? how many is too many? [speed vs energy efficiency]
-likewise the 'advanced machines' add-on requiring constant power to maintain the speed buff.