But I do agree this was very annoying would lose 20mins of work every couple hours working on stuff

No longer had the problem after getting a new laptop:
Ryzen9
GTX-3070
24GB RAM dual channel
This is probably due to unity being heavy on cpus especially with physics engines. As you add parts it has to recalculate pretty much everything. I have no problem building and running stuff but thats bc i have a ryzen 9.

it also happens to me... in my desktop, but i receantly got a new laptop and so far its not happening, i am working on a new big project as always and ill uptade if it happens again.

Desktop
I5-9600k
gtx -1070
8gb ram

Laptop
i7 10500
rtx 2070
16gb ram

@ReinMcDeer same here, especially it crashes a lot when i open connections, wanna change the size of the fuselage and when i open overload. This is annoying af, but yeah, not unplayable

I have the same issue, especially since the last update. For me, it seems to be triggered when I'm connecting parts, deleting parts, loading subassemblies, editing xml values, but not when I'm flying around.
I also click around really fast when I build, so maybe that has something to do with it?
It's pretty annoying, but not to the point where SP is unplayable.

@BMilan hm...

@Tetrodotoxin numerous times

@BMilan huh, interesting stuff. Did you try to reinstall the SP? Maybe it can help? I don't know actually, just guessing

@Tetrodotoxin I don't think it's because your laptop is weak.
I remember that this problem started after the 1.10 update and I read it somewhere that a similar problem can be caused by a specific Unity update so it might be that Jundroo updated the game's Unity version in 1.10 and that caused some compatibility issues with certain systems.
for some time I suspected that mods might cause the problem: but I tried disabling every single one of my mods and the problem still persisted. Now I have the same mods enabled on both of my laptops (Mod Settings, DesignerSettings, DesignerSuite, Color Editor, Overload, Fine Tuner, Underwater Camera, SP Multiplayer) and the game crashes upon alt+tabbing on the old laptop while it doesn't on the new one, it's a weird thing

@BMilan my laptop is pretty weak
i5-3320M
Intel HD 4400
8GM RAM
OS Windows 10
Maybe it's the problem, but it can crash even when loading 90 part build i dunno

same here but it crashes because of custom munitions

I've been fighting with this exact issue on my old laptop too. never found a solution but it is not happening on my new laptop
Would you mind sharing your PC specs? We might be able to find a solution
My old laptop's are:
OS: Windows 8.1
CPU: i7-4720HQ
GPU: Nvidia Gefore GTX 950M
RAM: 8GB

New laptop's specs:
OS: Windows 10
CPU: i5-11400H
GPU: Nvidia Gefore RTX 3050 Ti
Ram: 16GB