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I'm sorry but, this is the worst possible example to judge lens performance from :-) There's nothing but stars and the stars are smeared because your exposure is too long. We need to see a picture of a static subject, not the ever moving universe ;-)
But at first glance it seems normal, I've shot countless starscapes with a Samyang 14 and your shot feels similiar. Normally the stars at the edges are more stretched than center ones.