I think what you have put here is too cryptic for a definite answer. However, placing a WRITE is likely to act as a barrier against optimization or dead code deletion, not to mention that WRITE itself is slow.

I think this is the explanation: Your code computes "dCRy" in the loop but without using the value computed in the loop, you re-define this variable in one of the 6 lines you refer to. Thus it is a valid compiler optimization to remove the loop completely. This then accelerates your code of course. In case you remove the second definition of "dCRy", the loop is non-redundant and has to be executed.

Yes - right. The compiler consider the loop as 'dead code' since all you compute here is never used. You will see the change in case you add e.g. a
WRITE(*,*) dCRy
after the loop: This would be a very simple test to verify the change in run time. Or compile once without optimization ( -O0): Then the compiler would not remove the loop either.