This is honestly such a huge problem for very quick missions, most notably void fissures and I know I cannot be the only person that has problems with this. It seems like ever since Warframe went completely cross platform, loading into a mission takes longer. This isn't just a "me" problem, I see players stuck in the loading in state for huge amounts of time in a lot of missions.
As I said earlier, this is a massive problem for very short missions like void fissure runs, because sometimes a player won't be able to get into the mission until the rest of us are already at extraction, and time is just wasted waiting up to try and get everyone up to 10 goddamn void traces. Some players aren't willing to wait at all. It's just a huge drag on something that used to be really quick and easy.
Usually I'm not bothered by technical wait times like this in games, but by design Warframe demands quick connect-and-go missions, so connection issues absolutely have to be worked on. Either that, or the mission objectives themselves need to be more lenient to reflect late joiners (such as dropping the void fissure requirement)