Ninebot es2 (with external battery) won't go past 15km/h. Help

Hey everyone, so the title basically says it. I've used it to commute a couple of times but had to put it to the side last week because of the weather (I'm not risking hard tires on wet asphalt ahahah) but when I tried it yesterday it wouldn't go past 15km/h on flat terrain when before it got to 20km/h without any problems.

It is in sport mode, so it isn't speed capped.

Anyone knows how I can solve this?

EDIT: it's ninebot es1, sorry!