Without any information about your system, it could be a near infinite number of potential reasons. Of the top of my head we could be dealing with Driver issues, If you have dual GPUs (integrated/dedicated) you could be defaulting to the integrated GPU. There could be corrupt settings with the GPU in Windows or With the game client itself, there could be any number of programs running on the system that can affect GPU access or performance within the game client. There could be permission issues or other OS level instabilities at work here.
You are welcome to try anything you can think of related to the above but if you are looking for more direct community support it would help if you could provide us with some information as mentioned here.