FPS issues

Hi guys, im currently facing an issue with gta 5 fps, when I start up the game i go into gta 5 story mode and the FPS dont go above 20, as soon as I change to online FPS are normal and great, 90-120 depending on where in the map I am, as soon as I go back to story mode FPS are normal as if nothing had happen, the thing is, if I want to add any mods so I can play by myself evidently it wont allow me to join online and back to fix it, does anyone have any clue as to why when the game loads up the FPS are that bad. ( its important to add this happened with everything up to date as to my graphics drivers and software and else...) I just bought the 4060ti so graphics and computers specs are up to date, so thats not the issue).

I have uninstalled the game with the games folder and reinstalled everything from scratch, its still happening. My issue is, i know it can be fixed by going into online mode and return to story mode, but as I intend to play lspdfr I cannot join a public online lobby for obvious reasons therefore the problem as to what can I do, does anyone know how to fix this? 

Thank you guys