I forgot one other thing for modellers, in particular infantry & aircraft modellers.
Weld vertices where possible. Extra vertices requires extra calculation, which means more system resources are wasted.
This isn't too much of a problem for vehicles but it is a major problem for infantry or any other models that use the WWSkin modifier. Try breaking your infantry models & get a large number on screen, then try the unbroken models. You'll get a lot more unbroken onscreen before you notice significant lag.
For aircraft it's a different story. Unwelded vertices still cause more calculation but more noticablely they can cause the "evil shadow bleed through" problem, but there's a catch. If you do this to a poly & make it 2-sided* then you create another shadow bleed through problem. In this case you weld what you can or just don't worry (a few here & there isn't the problem in aircraft like it is in infantry)
*2-sided (welded together but still 2 separate polys, not the geometry export option)
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