I just made a comparison on GNU/Linux gcc (precompiled Qt for GNU/Linux gcc) vs clang (Qt compiled with clang on GNU/Linux).
The execution time is about 10% faster with FET compiled with clang :) (like 6 m 4 s with gcc and 5 m 29 s with clang).
(the compilation time is faster on gcc: ~2 m 34 s gcc, ~ 3 m 20 s clang, compiling on 16 threads / 8 cores).