Personally, being on an older device (Snapdragon 660, 3gb ram 32gb storage) SP can be heavy to run after 460+ parts, and completely unplayable at 615 or more, other things that lagged the device out are long FT codes, image labels, and XML upscaled parts

Part count doesn't really matter, although around 700 parts is mobile friendly, 800 parts is also mobile friendly even if the download button is yellow
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Size matters a bit, if I make an aircraft 8x larger than 1:1 scale it lags a bit but in the designer it will lag a ton because of how big it is
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Performance cost gets higher if you add more parts, fuselage pieces are 2.4, hinge rotators are 19.5 I think, the performance cost for a mobile friendly craft is I believe 4000-5000, other people might be able to handle more
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Edit: as a previous comment the wheels does have a high performance cost, the labels not that much although if it has lots of codes in it it will lag the game a bit

My max on a android is 3600 parts with medium lag (still playable though)

Anything made by Jamesplanesii...

yeah it's mostly part count, size is much less of a concern
however, the one that really lags is labels being used as images, those will absolutely shred your FPS
it should also be noted that all 3 helicopter engines, as well as the BFE150 (afterburning jet engine) have rather high performance cost due to their complex animations, so avoid using those if you wanna keep it down. rotators and wheels also have a decent affect on it but nowhere near as much as those engines.

@Adelweis Device hardware (RAM, CPU, GPU...)

@AnOmalOcaris dont forgot the FT spices, it adds up the weight

@AnOmalOcaris what is hardware?

Drag, craft size, hardware and performance cost are the causes of lag