if the device switches off while in use on charge, that's too slow. if the device switches off whilst charging because it overheats, that's too fast. in between, forget it, the electronics will handle everything, and yes slower charging is better for battery life but you needn't obsess over the difference, it's there but unquantifi
able. if you've heard anything other than this advice and you believe what you've heard, there's not much point in people here trying to guide you.
the phrase is "waffle stomping"