Performance degradation due to Auto Vectorization

Architecture:          x86_64 (Haswell with 6 cores)
Compiler Version: icc 15.0
Performance degradation while compiling with autovectorization(-O2) on the code snippet below:

#define N 200000
void foo()
{
	__declspec(align(64)) int a[N];
	int i,cnt=0;
	for(cnt=0;cnt<1000000;cnt++)
        {
		for(i = 2; i < N; i++)
		{
			a[i] = a[i-2] + 1;
		}
	}
}

Compilation method 1 with vectorization: icc -O2 <filename> -opt-report5
Result: Time taken (3m 24 sec)
Report says for loop above is getting vectorized with estimated potential speed up of about 1.2

Compilation method 2 without vectorization: icc <filename> -opt-report5  -O2 -no-vec
Result: Time taken (1m 08 sec)

Why is autovectorization degrading the performance even though estimated potential speedup is 1.2?