Issues with lagging and freezing can come from overtaxing your device. If you notice that your issues worsen after prolonged use of your device, this is most likely the issue. If you're touch screen is responsive enough try closing all running apps and programs. For alternative solutions, check here for our troubleshooting guide on screen lag.
If your problem persists with no apps running, it's more likely to be your touchscreen is not responding to your input. Check to make sure your fingers and screen are clean and clear of obstructing substances (screen protectors may be causing the issue). Additional information regarding this is posted here as well.
Let us know if your issue requires further assistance.