I had been told that g95 is not as efficient as other Fortran95 compilers, but I have been amazed at today's experience.
Job times for my simulation with identical parameters on the National Grid service:
g95 compiler: 2762 s
ifort compiler: 468.3 s
This is a factor of 6 difference!
I wasn't doing anything clever with either compiler. Presumably both compilers can be tweaked in their performance, and the gap in performance narrowed.