@linuxgurugamer
Two people in the past (@Watchboy52 @WhitestWizard) have posted about DangIt FPS drops, I was even messaged by someone (on the forum about troubleshooting the 'FPS drops with certain parts in the editor but not in game'; he later figured out it was DangIt.
I am also having the same issue.
The parts usually seem to be parts that are cargobays or specific command pods, but the only overlap I can find is that all offending parts contain the DangIt resource SpareParts.
The FPS drop is not consistent with a memory leak or anything usually seen with having a huge amount of mods, not only does the problem not persist in any scene besides the editor when the parts are in-scene, they go away as soon as the part is no longer in-scene (i.e. either a placed part or an unplaced hologram).
For example, an experiment I did was to place every command pod in a scene. Despite the FPS drop being noticeable within 3 parts placed (to the point that the video was no longer smooth at ~10 fps), after placing every single pod my game was running at 1 FPS and it took a few seconds of holding down LMB for a click to register. Doing this same experiment with a part that does not contain SpareParts will have no FPS drop whatsoever, despite the SPH being filled with a sea of Mainsail engines (for example).
In fact, sorting the parts by resource (Advanced>Filter by Resource>SpareParts), any part in this menu will cause the FPS drop problem, while any part that isn't in this menu will not.
Stock parts I can 100% say have an effect:
N.B. I have tested these and they are all affected by this bug, although some parts have more of an impact than others (this may just be placebo, as you're more likely to put down multiple cargobays and only one pod for instance, so you may not notice that it's also the pod)
All cargobays
All manned command pods (cores do not carry SpareParts and so do not affect FPS)
Can provide log if you'd like