Overall, in many cases from my experience, on Linux, ifort and gfortran have similar performance. ifort usually 10-20% faster, but the difference are not very big. However, it seems on windows, with the same flags, gfortran can be 7 times slower than its performance on Linux. Intel’s performance is consistent on windows and Linux.
You may begin with the following flags.
For ifort,
-O3 -xHost
For gfortran
-O3 -march=native
Both can be used with perhaps one single flag,
For ifort,