Note that this is very much compiler-specific (the person asking the question didn't say which compiler he was using), and I did not get the same slowdown in the Mono compiler (with or without LLVM).
if (i > 0)
value += 2;
value += 3;
value += i > 0 ? 2 : 3;
In one case, you create two different += operations with constant values and which one you
pick depends on a condition, and in the other case, you create a += where the value to add
depends on a condition.