There's definitely something wrong here. The i5 6400 can even utilize a GTX 1050 Ti to its full extent in Overwatch at 1080p, as can be seen in the i5-6400 run of this video(starts at around the 14:00 mark):
https://youtu.be/05ovUyNQpoc
Open Task Manager, and in the Performance tab, check the clock speed of your CPU. It should be listed under the 'Speed' section. When idle, the clock speed should be around the 2.7 GHz range. While under load, it should temporarily increase to as high as 3.3 GHz.