Testing optical performance of lenses requires knowledge and an optical laboratory. Subjectively, looking at the results obtained via the images you have captured, which may raise your index of suspicion about the performance of a lens so tested; may only point you in the general direction of disappointment with performance. In general terms, it is usually the case that a prime lens will outperform a zoom lens.

The large aperture zoom will have had to sacrifice some aspect of lens performance in order to achieve f/1.8 as a maximum aperture. I don't know the lens nor do I pay much attention to numbers published. Test results may give you a guide as to what to expect but they are usually results obtained under a specific set of conditions by one reviewer. The general applicability in real life situations may not hold up. Try to test lenses which you want to buy; first. Hire them for a week and see how they fit with your own techniques and equipment. This is far more useful than reading lab numbers.

Possibly your copy of the lens is faulty and informing the vendor is the first step in initiating a remedy. You can still hire an identically specified lens and see if it performs to your expectations. Check lens reviews on technical sites and try to assess the lens you want to buy from several different impartial technical reviews. If you still cannot resolve the issue, reject and return the lens and wait until you can find another lens and specification, which will suit your future needs.

The zoom lens can be poorly assembled and poor element collimation would produce symptoms like you have described. It is not your job to test your purchase. You can return it as unfit for the purpose it was sold. If a second and or a third lens produces similar results, you may have to accept that this particular lens cannot perform to your expectations. It may be time to rethink your lens collection. Given that you can already cover the focal lengths the new zoom lens covers, you may consider buying a lens which shows a different perspective.