I see your point, Stefan, so let me talk about our scenario here in Brazil so that it could help you analyze the case:
95% of our internet service is proved by 6 (yeah, only six) ISPs: VIVO (just acquired GVT, which was another ISP) / NET / TIM / Sky / Oi / Algar Telecom. (outdated source in portuguese: https://techinbrazil.com.br/provedores-de-internet-no-brasil)
Here in Sao Paulo, VIVO, NET and TIM are the most predominant ones, they are responsible for 98% of the market in this region. This region is where the server is hosted.
I play the game with two other people. My ISP is NET, and other two are VIVO (our connection speed goes from 30 mbps to 200 mbps, but it’s irrelevant to this case). We suffer from the exact same problems as most people here.
Outside the game, I usually ping the servers 30 ms any time of day, but inside the game, the ping goes all around: from 30 ms to 400 ms. Yesterday was unplayable: unstable 300~400 ms, never getting below that minimum value and skills were not functioning properly (as a buffing character, I usually get stuck in animation while the skill buff is not activated, and sometimes the skill fails but the cooldown triggers).
I’m not saying that the ISP are totally innocent here, maybe a fraction of this issue is regarding to them, but for us, considering this scenario, it’s clear that the servers are not good enough to handle this amount of requests. The game is not network optmized too. Maybe each client sends too much information for the server to process, and actually, that seems to be the case: the servers are not handling this much process very well, and lag spikes occurs because of this.