There are several possible reasons. Start with correct installation method. Did you right click on the installer file and choose Run as administrator? This is required for proper installation. See: Why the installer must be "run as administrator"
Next, make sure that your Radeo GPU drivers are up to date. In SU2016, you could offload graphics rendering to the CPU (turn off Hardware Acceleration) but that’s not an option in SU2017. Now you must have a GPU that is up to the task.
Finally, there have been numerous reports of sluggishness caused by some bad Windows updates. There are threads about this here on the forum. You may have one or more of those problem update files installed and need to remove them. See this thread: Selection Tool process is slow, unbearably slow