I got faster (10-20%) results with icc on an abstract game minimax AI bot back then (i.e. something similar to a chess engine). Even more so when taking advantage of PGO. Over time GCC caught up.

By nature, this code had no usage of floating point in its critical path.

I haven't bothered with icc in years though.

For what sort of application? I ran benchmarks of my own scientific code for doing particle-particle calculations and with -march=native I could get 2.5x better performance with Intel vs GCC.

One thing I found that you do have to be careful with though is ensuring that Intel uses IEEE floating point precision, because by default it's less accurate than GCC. This causes issues in Eigen sometimes, we ran into an issue recently after upgrading compiler where suddenly the results changed and it was because someone had forgotten to set 'fp-model' to 'strict'

If Intel is using floating point math shortcuts you can replicate it with -Ofast when using gcc.

It goes without saying that you should use -O3 (or -O2 for some rare cases) otherwise. I am mentioning it just in case because 2.5x slower sounds so exotic to me that the first intuition is that you're omitting important optimization flags when using GCC. GCC was faster than Intel on everything I tried in the past.