DIAGNOSIS This is typically one of two things: a temporary software process hogging resources, or a screen that's just too dirty, especially with oily fingerprints, confusing the capacitive sensor.
FIX Start with a proper clean. Unplug the device. Use a dry, soft microfiber cloth (like for glasses) and firmly wipe the entire glass screen in a circular motion. If there's stubborn grease, put a tiny bit of distilled water on the cloth—never spray directly on the screen. Dry it completely. Plug it back in. If it's still laggy, perform a restart. Press and hold the volume down button on the back for about 10 seconds until it restarts. This clears out any memory hiccups.
IF THAT DIDN'T WORK If cleaning and restarting don't help, there might be a problematic app or setting. Boot into Safe Mode to check. Press and hold the volume down button while it's starting up until you see "Safe Mode" in the corner. If the touchscreen works perfectly in Safe Mode, then a third-party app you installed is causing the problem. You'll need to uninstall recent apps one by one to find the culprit.
HEADS UP Never use window cleaner, alcohol, or abrasive cloths on the screen. The oleophobic coating that resists fingerprints can be stripped off, making the problem worse permanently.