Maybe I'm showing my age, but if seeking out opinions on cars I will often visit sites such as Autocar and TopGear, who in the case of the GT86 mention the words "pleasing level of performance" and "usefully brisk" .
Have not seen the term "insanely slow" anywhere, perhaps I should be checking out Top Fuel Dragsters Monthly? 😃.
On the right road a GT86 can most certainly feel pretty fast and is hugely more involving than a FWD VAG diesel (I've driven quite a few over the years - and don't especially want to ever drive another......) Twisty rural roads are where the 86 really shines, but the car for me is simply an enjoyable drive at any speed on any road - it's always a great place to be.
Something to bear in mind that might be relevant for your roadtest, an 86 drives better solo on Vpower than with a 15 stone salesman sat next to you and 95ron E10 petrol in the tank .....
In the meantime suggest you watch this (even if you've already seen it)