This has been discussed so many times already on the forum and, of course, GSX has a solution, explained at Page 26 of the manual, the explanation of the
Passenger Density setting, which I'll quote here:
Since passengers are now no longer simulated, but will appear as actual animated characters, the original timings in seconds/passenger will not be meaningful anymore: the time required to board passengers will be a combination of their walking speed and their density. Each passenger has its own walking speed, which depends by its animation and walking style, but it’s possible to control the Passenger Density, which will affect the number of animated passengers visible on screen at any given time.
Setting the slider towards the highest density settings will shorten boarding and deboarding times, but with some cost in performances.
Of course, the kind of parking you use has an effect on it as well.
If the parking is in the open and you use a bus, with 3 stairs, the passengers will board the bus much quicker than a gate with a single jetway.
If the airport is done realistically, with double jetways in their proper places (heavy gates), the time will be cut in half and, if the airport is not done realistically, still using default single jetways everywhere, you CAN replace them with GSX Level 2...
a) a number of passengers in a specific plane
This one is instead explained at Page 29.
Estimate passengers number
If this option is disabled, GSX will always ask confirmation from the user about the number of passengers onboard. If the option is enabled, GSX will estimate the passenger number automatically, without asking confirmation. By default, the option is enabled, which mimics the standard behavior GSX always had until this version.