Everyone who experiences the FPS issue with LSPDFR when on duty , then make sure you run the entire game from SSD or fast HDD ... I personally just tried to install GTA5+RPH onto an external SSD where I even managed to install the Social Club with it and at the first launch it loaded perfectly.As the RPH proofed its working , then I gone back to windows added ELS and some new cars plus installed LSPDFR 4.6 manually (no callouts yet) and launched the game via RPH again. As the game loads smoothly I load LSPDFR via console and going to a station for the first duty to set my character.I made sure the graphic settings were boosted to the highest possible at 1920x1080 , so far the game still at 60FPS(FPS locked with V-sync) , finally pursuit callout came trough so I rushed towards the suspect and called for heavy back up in order to seek for any FPS drops.at least 10 cars arrived in my aid and the game never left 59-61 values on the FPS counter.Also tried to go all over the county while still on duty and receiving callouts without any frame issue.Also tried to turn off V-sync to see the real values that the game is managing to operate on and it was something between 75-115.
I hope that helps at least to some of yous as I personally been pretty annoyed of having 2060 , with 8 core 4,2 and 16GB ram and getting lags on a game from 2013.
Stay Safe!