Cheap chargers can interfere with capacitive touchscreens because cheap chargers have a lot of electrical noise in their output. This noise gets coupled to the touchscreen and causes problems. Specifically, the touchscreen controller needs to measure pico-Coulombs of charge which is difficult when there are multi-volt spikes coming out of the charger. The chip can perform various filtering to deal with this noise, but that slows it down. Some touchscreen chips even do frequency hopping to avoid the noise (kind of like a military radio trying to avoid jamming).

Details here: https://www.cypress.com/file/117656/download