Same issue here. Especially in campaign map If I use fast forward my fps is dropping from 60 to 40 for a split second every couple of seconds. Same in every graphics level. 1080p, 900p, high, medium, low.
my specs:
OS: Win 10 64 bit
GPU: GeForce GTX1060
CPU: Intel i5 6500
RAM: 16GB 2666mhz
Storage Device (HDD/SSD): OS on SSD, game on HDD
In-game graphic settings: tried every setting on 1080p 900p windowed and full screen. Same high loading times, same stuttering, same level of performance. My in-game refresh rate is also locked at 50hz for some reason.
I am reverting back to 1.5.5. Maybe it'll fix my problems.
UPDATE: Changing to 1.5.5 solved my problems. Clearly 1.5.6 has too many performance issues. For my case the performance issues on campaign map whether it is travelling, going into towns, opening tabs(quest log, inventory etc.) were actually made the game impossible to play. Going 1.5.5 also solved the 50hz lock on video tab. I can now play the game on high settings @1080p60fps without any problems. I didn't test the bigger wars of course. Only played against small groups of bandits and looters.