Issue with Photoshop CC on imac when I use large size brush

Hi, I recentrly bought an imac and I installed Photoshop on it. I was trying the performance and something strange happened; when I use a large size brush/eraser ( or other tools that require a size ) the whole monitor become black for a moment every time I move the mouse or I push the space bar and once it crashed and I had to restart the imac. My imac has a 4k resolution, i7 processor and intel iris pro graphics 6200. There would be a way to solve this issue? I thank you in advance.Iss