Increased Latency only in instances

Hi fellow Warmane players

As title says that i have a little problem with latency
I am aware that my latency to warmane servers are not great (around 200 - 350ms on normal) which is pretty good for me assuming that warmane servers are overseas from where i live.
The problem is when i enter an instances (dungeons or raids) the latency gone up but only when in combat (1 - 3k ms even worse), after that it takes 5 - 10 sec to back to "normal".

My question :
1. Is there any reason why this happen ?
2. any tips or suggestions on how to fix it ?

Thank you in advance
Best Regard

PS : Sorry for the bad english