If you are using a laptop with NVIDIA Optimus technology, you must force your computer to use the NVIDIA graphics card. See NVIDIA's support article for assistance.
Try running the game in a window and with no sound. You can enable windowed mode in the in-game System menu by changing the graphics Display Mode dropdown to Windowed. You can disable in-game sound by unchecking Enable Sound in the system Sound menu.
Try selecting a lower setting from the Sound Channels dropdown in the System > Sound menu. Note: Some add-ons may set Sound Channels back to High after installation.
Errors on your hard drive could be contributing to your entire computer slowing down. Run ScanDisk and Defrag to address hard drive issues.
Crashes and lockups may be caused by problems with your computer's physical memory. Run a memory scan to rule out this issue.
World of Warcraft requires an up-to-date operating system to work properly. Run Software Update.