If I had to take a guess I would say that it’s probably because of different default variables between the two compilers.
There are some very experienced gfortran devs here that might be able to shed some light into this.
As for the difference in per thread performance, I would expect it to be noticeable. Intel’s MKL is an absolutely amazing library that can significantly speed up calculations.
Although I think you can still link to MKL whilst using gfortran