Execution time speed compared to ArrayFire C++

Hi,
I have developed a Arrayfire image processing algorithm in C++. The execution time was around 78-80 ms. Then, I re-implemented the algorithm via the .NET wrapper. Now the execution time is around 120 ms. Is it normal?

PS: I also noticed the .net wrapper leverage 50 of the GPU however the original one used 100 percents
One more thing, the .net wrapped version has no "warm up" time however the original C++ version needs about 1 min to execute for the first time. I am not sure yet if its normal or not!