As far as i know this useless stupid folder has purpouse to speed up loading stuff in game,right ? if so,IT ABSOLUTELY DOESNT IN MY CASE,In matter of fact,if i clear cache and sometimes wtf/interface folder it loads game in like 5 secs.The longer i have this stupid cache folder the more time it takes to load stuff.
Are you by chance using addons that are likely cache a lot of items/achievements? AllTheThings, Altoholic, TradeSkillMaster or similar addons?
Because they tend to slow down your game after a while of use. Sure it goes quicker when you just boot up but as soon as you open them up they start loading(and caching) data.
Sorry to hear that your games loading can slow down as you play. As you mentioned the main purposes of the cache file is to help you obtain game data more quickly to help speed up areas where you play in the game.
Konata covered this well in their reply too. The cache file shouldn’t increase loading times and if this is occurring it is likely a combination of addons/corrupted UI elements or background applications interfering damaging the cached data.
Properly resetting the games UI and closing any background apps as possible will likely help here.