Hey Andrew,
One reason why you're touchscreen is slow may simply because your screen is dirty. If this is not the case, it may be a circuitry failure. Try rebooting your phone by turning your phone off for 20-30 seconds and then turn it back on. Your phone should start working normally.
For more information on unresponsive touchscreens, check out the [[LG Power Troubleshooting|troubleshooting page]].
Hope this helps!