I would guess people who suggest the newer one's aren't as fast as the 2015 could own a 2015 and are trying to justify it.
In truth, the newer machines are significantly faster on paper, as anything newer tends to be. In real term usage, they have always (All Pro's) been significantly faster than what most people actually need, so the sheer headroom in performance that wasn't being reaching on a 2015, just has a larger headroom on the 2017. But they're still no where near crippling the machine in performance.
So yes they are faster, but whether they are noticeably faster largely depends on your current use of the machine and how old it is. If you find your current machine perfectly fast, then you will not notice any speed increase (You can't make something that is already fast, faster...). However, if you find your current machine slow and sluggish then you are probably at the peak performance and would benefit from an upgrade.
I would go as far to say that anyone asking these kinds of questions likely does not need a new computer, usually if you need a new computer you know it, and you know what to get. If you are concerned about performance and stuff then you probably work in a field where this is a priority and should professionally already know this stuff. Other people reading random reviews or looking at 'specs' and determining they 'need' something or don't, are simply speculative. So don't worry.
Buy what you need, the best you can afford, when you need it. Simples.