I’m having this really weird issue when I’m experiencing lag, but only with spell casting, and specifically only spells that have a cast time. There’s (what feels like) a 1 second lag between when the cast finishes and when the spell ‘goes off’. This lag doesn’t affect my instant spells (those go off and hit instantly) and doesn’t affect other actions that have a cast bar like skinning or summoning my mount.
I transferred my character to Grobbulus and I didn’t have this issue before I transferred on my old server. My latency is consistently at 80ms on Grobbulus. I’ve disabled all of my addons and the cast lag is still there. I also reduced my spell queue window to 130 which didn’t help. I last ditch created a new character and the new character’s casts are not lagging.
Also, not sure if it’s related, but my character is stuck in a permanent shooting pose. Closing the game, logging, etc hasn’t fixed that either. 
I’m having a really hard time figuring out what is causing this hyper situational lag. Any advice would be greatly appreciated!
EDIT: Turns out it was the shooting pose that was causing the spell lag. Shooting pose was caused by quest in Nethergarde Mine where you’re sneaking around in a crate. Fix is to die and recover your body. This fixes the casting lag.