Which components actually make a difference?

Depends what "performance" you are talking about.

Good track hubs will spin with less resistance than cheap road hubs due to lack of seals, better bearings, better machining.

The lack of lube, lack of seals certainly won't 'perform' well in the outdoor durability stakes though will they?

Performance could mean all manner of things, not just going faster. Everything is a compromise anyway.

I tried to be quite specific and I said that durability wasn't a consideration (don't fight the hypo!). Everything is a compromise as you say.

I'll put it another way. If you did a properly double-blind test with two identical bikes except for changing one component, changing which components would make you feel an immediate, **substantial **difference in comfort or speed?