Dramatic FPS drop in world zone.

Hello warmane community , I am facing one stubborn problem , everything was fine 2 days ago but then my fps dramatically drops in world open area , when I am in some house or dungeon fps is on the place.
I have deleted WTF,cache,interface folders from WoW folder , everything is okay until I restart computer,then it's same.
I have all drivers up to date , same for addons , reinstalled win 10 , still not working.

My computer spec : Graphic Card- GTS 250
Processor: Inter(R) Core(TM)2 Quad CPU Q9400 @ 2.66GHz 2.67GHz
Windows 10 x64bit - x64 based processor
4GB RAM
All games runs perfect except WoW Wotlk and TBC.
No background apps while playing.
On my other computer WoW works perfect with same addons activated.

WotLK and TBC use single thread/cpu, can't use more. That's why they work a lot worse than other games. I have well, you can say an OP PC (Black Desert rammed on 60 FPS on Ultra) and yet I still get heavy drops on WotLK in AV, cities and WPVP (when there's 40v40 or more). It was a lot better when I had it on SSD a long time ago, but now I have it on HDD.

You can reduce it only by turning shadows off, lowering particle detail or moving it to an SSD.

Actually you can edit your configuration file to use the other cores, but by default it does indeed only use one core. There's even an old guide here on the forum telling you the steps if you do not want to go digging through old forum posts on the official blizzard forums http://forum.warmane.com/showthread.php?t=310635

Yeah as the guys said, having wow on SSD will speed it up, on wotlk and tbc shadows, well just turn them off they do barely anything. For older pcs you can change Nvidia settings too to get extra FPS, I can show you how to do it if you want me to (send private message) also as Triska mentioned about cpu's, that's something you might want to do aswell.

First , thanks for response ! Second , nothing works from above (about CPU's) , it's kinda confusing , sometimes when i open wow it works perfectly like a charm , and when i loggin second time I have low fps 20-30 (usually 60) , yesterday i was playing League of Legends and it wasn't smooth as it should be , what is slowing down my gaming experience?
Fresh windows 10 , all drivers up to date , no addons, no background programs..

Have you cleaned pc from dust? In inside of pc that is. It may slow it down.

Reinstalled Windows 10 again , and now it works like a charm , thanks for responses!