Disconnect any unnecessary USB devices from the computer while using the keyboard. Every device that is connected to USB can use resources on the bus. If you have a lot of devices connected, then it could slow down the bus. If you had been using a USB hub to connect the keyboard, then you should instead connect directly to an onboard USB port on the computer itself, rather than connecting through a hub. You can also reduce the amount of system resources used by the operating system if you close out of any non-essential programs that are running in the background.
If the above recommendations do not work, then the problem might be related to the computer's sound hardware. In that case, installing the ASIO4ALL universal ASIO driver at http://www.asio4all.com might help. (Note: ASIO4ALL is not Casio software, and it is not something that our technical support group would be able to assist customers in using or installing.)