OK have finally got Nanite to lvl 5 so here are the build times between having a (Nanite 5 with Robotics lvl 12) and then lvl 13
Light Fighter (3 secs) - No change
Heavy Fighter (8 secs) - No change
Cruiser (23 secs) - 21 secs
Battleship (51 secs) - 48 secs
Small Cargo (2 sec) - 3 secs ?! - Strange but true!
Lrg Cargo (10 secs) - 9 secs
Colony (25 secs) - 24 secs
Battlecruiser (1 minute 0 secs) - 56 seconds
Bomber (1 minute 4 secs) - 1 minute 0 secs
Destroyer (1 minute 35 secs) - 1minute 28 secs
Death Star (2 Hr.09Min 48 secs) - 2 Hr. 0 Min 32 secs
Recycler (13 secs) - 12 secs
Solar Sat (1 secs) - No change
Still a fraction of the difference gaining Nanite 5 made but, for such a low cost, well worth the upgrade if you have spare slots.