0.12.24 - New Hope Mission 2 - Game slows dramatically

[Ursus]

After completing the Automated Rail Research and getting the new mission objectives to collect 2500 armour piercing rounds etc, the game slows dramatically. Changing the settings doesn't make any noticeable difference.
I have only started playing the game so I am not sure if it slows down in free play. I am guessing this problem isn't necessarily linked to the mission, but rather an increase in processes that happen after reaching that milestone in the mission.
I am guessing this problem will happen when I start playing multiplayer and in the later stages of free play so any suggestions on how to resolve it would be appreciated.

Operating System: Windows 7 Home Premium 64-bit (6.1, Build 7601) Service Pack 1 (7601.win7sp1_gdr.151019-1254)
Processor: Intel(R) Core(TM) i7 CPU 860 @ 2.80GHz (8 CPUs), ~2.9GHz
Memory: 16384MB RAM
Available OS Memory: 16380MB RAM
Card name: NVIDIA GeForce GTX 960
Display Memory: 4095 MB
Dedicated Memory: 3072 MB
Shared Memory: 1023 MB
Current Mode: 1920 x 1080 (32 bit) (60Hz)

Thanks.

[Koub]

Your PC specs give absolutely no clue : you should have no problem running the game.
Could you please post the usual stuff (logs, eventually a savegame), so that we can try to reproduce and see in the logs if there's something wrong ?
https://wiki.factorio.com/index.php?title=Log_File

[Loewchen]

While slowed down, please make a screen-shot with show_fps, show_detailed_info and show_time_used_percent activated (F4 for debug menu, F5 to activate basic tab) and post the factorio-current.log and factorio-previous.log (You can find out where to find those here) as well.

[deepdriller]

Just discovered the issue. For unknown reasons, activating F5 fixes it.