FPS drops is normal, because your game is running a lot more data usage, has nothing to do with your PC, the game is limited to a certain amount of memory, some plugins run way harder on the system than others, I just went through and did this sequence to find the culprits.
#1.) Start Game - F4 - "forceduty"
#2.) Remove one plugin by Cut pasting to desktop,
#3.) F4 - 'Unloadplugin LSPD First Response.dll'
#4.) F4 'Loadplugin LSPD First Response.dll'
#5.) F4 'forceduty' test FPS to look for increase.
Repeat these steps with each plugin, or you can just remove them all, and add them one by one, when you're editing Plugins folder you don't have to restart the game, you only have to restart LSPDFR, sometimes you can just forceduty on and off and the non-standalone plugins will reset, but I prefer to fully reset LSPDFR. Once in a while I randomly get FPS drops, playing a little while and moving locations helps, forcing all the peds/cars to reset and the police spawned. Hope this helps, it's the way I look for FPS intensive plugins.