It's common for people to be confused about what desync is, what causes it, and how to fix it. If you're having performance problems and you think desync is the reason why, this is not correct and you should check out my other guides.

First and foremost, it's important to understand that desync occurs when your settings differ such that code executed by the game will - on a long enough timeline - appear in completely different states for each player. This could be differing cheats or it could be any Dolphin setting that modifies how a game actually runs - such as skip EFB access from CPU.

A desync occurs whether or not you actually experience weird gameplay results. Depending on what specifically is different, you may not experience the result of that desync for 5 minutes, 5 hours, or 5 days of playing. What people typically call the desync is actually the manifestation of it and not the actual desync itself. Things like what stage you're on may have been a contributing factor to the manifestation but it does not cause the desync. The desync was already there and you are now witnessing the effect.

As such, ping times, ability to play without dropping frames, etc. are all irrelevant to desync. It is only a configuration mismatch. One or both players may need to change their configuration to the standard configuration.

For example, suppose one player has the widescreen Gecko code and the other player does not. This is a very good example cheat because it's easy to manifest the desync. Everything goes more or less fine until the instant that one player goes into a bubble on one screen and is not in the bubble on the other screen. The instant that happens, one Dolphin is executing game code that the other Dolphin is not and it quickly changes the timing for input polling and everything.

Which is why you see the other player doing complete nonsense. You're both literally responding to differing game states and the game is also processing the inputs at different times which means that what you thought was a sideb might be an upb on the other screen depending on the timing of the poll. Anything like that.

Some cheats like my example cheat have clear and obvious ways to manifest a desync. Some cheats, such as the "netplay-safe" kill music cheat have currently unknown interactions in the game code that manifest the desync. It will happen at some point, but again, it may be 5 minutes, 5 hours, or whatever else before it actually causes a gameplay problem to the players.

The only way to avoid desyncs entirely is to have exactly the same settings as far as Dolphin execution is concerned.