Question...

I have had many different experiences on mobile. Depends on your device.
I had a $1500 iPhone 7 that wouldn't do more than 500 parts without booting me out.
Then I got a $100 Motorola and it will do up to 600 before lag, and 800 parts total.
My $4000 pc will do 5000-8000 parts before lag (60-120FPS) and it can fly the 12000 part futuristic freighter craft on the front page with 5-15 fps on a planet and 20-60 fps in space.
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Remember that not all parts are equal for processing information and G Ram.
The strut is the easiest on the processor as it doesn't send many calculations to your device.
A wing or moving part would be the greatest amount of calculations probably.
Vizzy can be hard on the processor as well.
So if a craft is completely full of high processing parts and lots of vizzy work, it makes things work a lot harder than another craft with the same part count but nothing but struts and a few wings.
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If you want to min/max your device, minimize graphics as much as you can stand and set physics to high.
That will make the planet and environment look bad but you can still test a higher part craft out.
Remove shadows from the designer also if its not already and be sure SR2 is the only app running.

Little bit of both? Idk, I've played on relatively low tier devices and still had minimal issues with higher part counts. Yeah, the game gets slow, but never enough to make the game unenjoyable. I've also played on higher end devices and can assure you that the game is just well optimized in general, since even ridiculously high settings with high part counts doesn't make much of a difference then