Having some seriously low FPS issues (dipping to around 10-20 FPS sometimes)

Hey all, new to this website and I'm pretty new to forums, in general, So I decided to post something I've been curious about for a long time.

Why is it that when I load into my games, I experience some serious dipping in framerate? (Example, the framerate drops everytime I spawn in a weapon that hasn't been spawned yet after loading into a new game [ I mostly play singleplayer or multiplayer on my own hamachi server with some friends ] or I'll shoot a weapon and the game could spazz out and freeze for several seconds). It's like I basically have to introduce every action the SWEP does step by step to the game, I.E., spawn the weapon, Experience Lag, shoot the weapon, lag again, and keep shooting the weapon until the game almost sort of "accustoms" to it and stops dipping in FPS when I use a weapon... I'm basically just asking if this sort of lag with spawning in weapons and NPCs and the likes is normal.

Also, I haven't introduced a new part to my PC since I had it passed down to me from my cousin back in 2014, and I'm not sure if it's my computer that's having so much trouble running the game or if it's just the game itself having trouble with mods. I don't even have many mods as well since I manage them quite frequently.

Any thoughts? Here are my specs:
CPU: Intel Core i5 CPU @ 2.80GHz, 2800 Mhz, Quad cores
GPU: GTX 660 Ti
RAM: 8GB DDR3