Unfortunately location doesn't have much to do with latency, it all comes down to how your ISP chooses to route your connection to the AUS or US servers.
Lots of SEA players play on Oceanic servers due to the timezone and I believe that would be the best fit for you. Now if you're getting high latency to Frostmourne, there's a good chance your ISP is the problem.
I've gone through several posts on the WoW forums and noticed that it's usually the ISP's fault. For example one player's ISP routes the connection through some US backbone which causes the connection to route first to the USA and then back to Australia.
If the latency you're experiencing right now is too much, you have two options:
- you can report this to the WoW technical forums and post a WinMTR. The CMs are really helpful and if your WinMTR provides any points of improvement for Blizzard's network, they will help.
- contact your ISP and provide as much info as you can (WinMTR) and ask them to correct the routing to Australia.
Unfortunately since each ISP works differently and latencies vary for US and Oceanic servers, it's mostly up to you test each server and find one with the lowest latency.
One last thing to note, since WoW works with sharding now, there's a good chance you will be ping-ponged around different servers - where while solo you might get 100ms, grouped 200ms (since you were transferred to the other players' server) and running a dungeon or arena game at 300ms.