Invincibility after being hit, besides being based on gameplay mechanics like using your sword, is indeed effected by framerate. Why? Because hit detection is clientside-based, and determined by what is happening on your screen. Instead of swinging your sword and hitting a player, then communicating with the server and telling it(and your victim) that you hit a player, it works the other way around. Your client is constantly looking around your player, and if it sees another player using their sword in a certain range, it registers you as being hit by them. This causes the whole "blink delay" issue because if you have a horrid framerate, your client is literally running slower than the other players. This means after you're hit, normally the client would ignore the hit detection for say... A second. But aince your client is runnig slower, that second of invincibility can turn into three seconds. This is why chat blocking is a thing, because the keyboard on the iOS dramatically decreases the framerate of Graal. Could be worse, PC graal lets you do certain things that completely freeze your client, making you invincible until you disable it. Like a convenient, in-game lag switch... Woo.

Sorry for the massive wall, on my iPad so typing is a pain.