Boarding: When I refuel via Fenix, aircraft is also loaded with cargo and passangers. In this case I assigned 153 passengers. After that I start boarding via GSX but it loads 100 passengers... Why?
Because you haven't read the manual, which clearly explains that since passengers are not something like a standard variable but a custom internal simulation in the airplane, GSX cannot read a value that has been calculated by a 3rd party airplane internally (not without the airplane having some kind of SDK that exposes that), so normally GSX will estimate the number of passengers from the Payload, as it always did for the past 10 years, which of course will never match the airplane own internal simulation, unless the airplane coded a specific GSX integration, telling explicitly to GSX how many passengers you are supposed to have.
However, if the airplane doesn't have a GSX integration, if you use GSX SIMBRIEF integration instead, and set the passenger number in the flight plan, GSX will use that figure so, assuming the airplane is reading Simbrief as well, they'll match the number, since they are both using the same flightplane.
And we can see the heads of the passengers walking inside the gate.
That's because you haven't read the latest manual or release notes ( Page 65 of the Manual ), which explain why, on 3rd party jetways that have heights different than default ( 4.60 mt ), passengers will either walk too low or too high, but there's a way to set these heights in the GSX airport profile. It's a data that doesn't exist anywhere else in the simulation, so there are no other ways to possibly tell GSX about the height of a 3rd party jetway.
Also, I loaded passengers very quickly via Fenix and it took half an hour to load passengers via GSX. How can it be a little bit faster?
First, you cannot obviously expect GSX would even match the speed of simulated passengers with actual animated passengers. Both because for the same reasons of their number, the fact the airplane is using an accelerated boarding is not possibly known by GSX, but even if it was, it would be wrong to adapt it to real animated passengers, since they look ridiculous if we had to accelerate them to match the airplane speed.
Here, the setting in the manual you failed to read is at Page 29, the explanation of the "Passenger Density" setting, which has a direct effect on the time it takes to Board, since the higher the setting, the more passengers will be displayed at the same time and, since their total number won't change, with more passengers displayed, the Time to Board will be way shorter.
And of course, if the parking doesn't have a Jetway, but use Stairs, Boarding time will be 2x faster than with a Jetway ( because MSFS doesn't support double jetways ) for planes using two staircases, or 3x shorter for planes configured with 3 stairs.
Preparing for pushback: Waiting the non-needed stairs to complete operation. More than 15, 30, 45 minutes... I could not start the flight anymore because it stayed like this forever
Not happening here but, if you could make a proper report, clearly explaining exactly what you did, we might look into it.