I tried it with 10 slots as well on factories... built them to 1500m2 (took almost 700hours if I remember correctly) then sold them... which took a few days also...
It did make some profit in the end but there was no fun in doing it... just frustration.

... but that's just me... I like the product manufacturing and researching angle of the game... there is more money in products than construction... and a whole lot more fun...
