Learn Hardcore Turkish: Uygulama kaydırınca takılıyor; siz çıktı alın, ben onay kutusunu işaretleyeyim. - The app lags when you scroll; you print, and I’ll tick the checkbox.

takılmak: to lag/stutter/get stuck (can be temporary).
donmak: to freeze (often a harder freeze). Choose based on severity: takılıyor (laggy), donuyor (hard freeze).

It’s the temporal adverbial suffix -(I)nca/-(I)nce/-(U)nca/-(Ü)nce, meaning “when/once/whenever.”
Breakdown: kaydır- (to swipe/make slide) + -ınca → kaydırınca = “when (you) swipe.”

Who is actually doing the swiping in Uygulama kaydırınca takılıyor? The app?

Not the app. In -ınca/-ince clauses the subject can be left implicit and understood from context. Here it means “when you/one swipes (the screen/list), the app freezes.” If you want to make the subject explicit and polite, you can say:

Kaydırdığınızda uygulama takılıyor (“when you [polite/plural] swipe, the app freezes”). If you want the passive (“when it is swiped”), use:
Kaydırılınca uygulama takılıyor.

Why kaydırmak and not kaymak?

kaymak = to slip/slide by itself.
kaydırmak = to make something slide; in phone/UX contexts it corresponds to “to swipe/scroll (something).”
So for swiping content with your finger, kaydırmak is the normal choice.

Could I say kaydırdığınızda instead of kaydırınca? Any difference?

Yes. Kaydırınca is colloquial/neutral and leaves the subject generic. is more explicit and polite/formal (it contains “your [pl.]” in the suffix), clearly anchoring the subject as “you (plural/formal).” Meaning-wise, both are “when you swipe.”