Bug/issue report for 1.86-RC2 (didn't get to test the following with 1.86-RC1):
Something about the Manmut's Guardian Shield now causes FPS to drop from ~60 FPS to <30 FPS after a minute or so of combat, followed by a further drop to ~15-20 FPS after the Guardian Shield collapses / the Manmut is disabled or destroyed.
Slowdown persists for the rest of the battle / simulation, regardless of the choice / number of opponents, fighter-spam being utilized or not. This eventually forces an 'End Simulation' or 'Exit Campaign' to get away from said slowdown.
Having said all of the above, my flagship is the Edengate from Tahlan Shipworks, which has a Temporal Shell-like shipsystem, but also includes a High Energy Focus (except applied to energy AND ballistic weapons) + boosts to mobility and flux venting (yeah, it's a salvaged endgame boss ship).
Addendum: Using the 'RemoveHulks' console command fixed the framerate issue, so I guess the (partially borked?) Guardian Shield script persists until the Manmut itself is removed from the battlespace?