Today's biggest problem with AMD graphics cards is RX 580 8GB. Because, RX 580 has actually 2306 stream processors. But now, all unknown brands are developing this GPU with 2048 stream processors. Because this GPU is cheap and it has nice 1080p gaming performance. That is why all poor brands trying to develop and sell this GPU. Therefore, developing is not being good enough and so many problems found with these new RX 580 8GB 2048 sp cards. And also this new GPUs have only one DisplayPort.
Maybe, your GPU is one of them.
Try playing light games with your iGPU, connect HDMI cable to your motherboard, not to your GPU. If lagging will not continue anymore, it means your GPU is the problem. Otherwise you need to check remaining part of your hardware.
I recommend you to play Counter Strike Source with your iGPU of intel i5 to check the lag. Because this is very light to handle and easy to notice any lag or problem
