Even on modern hardware and a nice, fast, SSD I find the load times for GTA Online are still somewhat excessive - and it's all the more frustrating when you're switching sessions or going between single-player and online. I've never really given much thought to the reason why this is the case, just accepting it as necessary, but it seems one modder just didn't want to put up with the waiting any longer.
The modder has managed to reduce the game's loading times by nearly 70% thanks to a fix they made to the game's code. It's not something players will want to try though, for fear of potentially getting banned - instead, the code from tostercx is intended to serve as a proof of concept that the loading process could be a heck of a lot swifter. The result is impressive though, with toastecx bringing his loading time down from 6 minutes to 1 minute 50 seconds.
Apparently, the issue is caused by a 10MB JSON file that stores an eye-watering 63,000 items within it - with the game running through the entire list. The game loads the item, then checks the list again, leading to a total of 1,984,531,500 checks according to tostercx - a heck of a lot. To compound matters, the loading routine apparently only uses one CPU core.
Apparently, if Rockstar wanted to fix this, it should be more than "a day for a single dev" to resolve - of course, this does not consider the fact that there's also beta testing, acceptance and rolling out, all pretty logistically challenging - but even so, Rockstar has no real excuses not to make these fixes, especially now it's been handed to them on a silver platter.
I don't expect that to happen though, unfortunately.