GSX Manual, Page 95, the paragraph named MSFS takes longer to start with GSX installed in the TROUBLESHOOTING Chapter, has some suggestions what you can do to reduce the increase on the startup time added by GSX, for example disabling Airport Service, which won't affect GSX proper vehicles servicing your airplane, but only the accuracy of the operators selected for default ground services servicing AI airplanes.
If it's not entirely clear for you what Airport Services do, there's a table at Page 8 of the manual which explains exactly what they do and when they are used, so you can decide to disable them if they are not important to you.
Normally, GSX adds about 2:30-3:00 minutes to the startup time, which is comparable to a similar product that adds lots of models in many livery variations like an AI package, and you can shave at least a minute extra if you disable all Airport Services files.
15-20 of startup time is unheard of but, of course, if you have an insanely huge Community folder, your antivirus scanning each and every file ( not just from GSX ) might cause that so, the first thing I always do when installing MSFS, is to add its entire CONTENT folder ( the one that "contains" the Community and the Official folder ) to the antivirus EXCLUSIONS, which will prevent the antivirus scanning over hundreds of thousands of files while they are opened during the startup.
