Pokémon Go is frequently failing to login on my iPhone, same for my husband (Samsung) and even when we can login, on capturing or randomly, gym battles (1hp) etc, the game will just stop responding. We have WIFI call/WIFI in general also enabled on our phones.
Now, we do have bad carrier signal at our home (but excellent WIFI), so easy to blame that, but… This is also the case should we pop into town where carrier signals are very strong.
However, (this may or may not get your Pokémon account banned), using Nox app player, I’m able to login no problem via my PC using the very same internet connection I use on my phone. I experience little/no connection drops and no client freezing, just annoying that I can’t ( or rather, choose not to for cheating sake) move my location in game.
Not sure why, maybe someone can poke into this as it doesn’t appear the servers are really all that ‘slow to respond’ and crippled on a frequent basis as is being advertised. It feels like the mobile platform in general has some kind of flaw preventing stable connections.