Tree of Savior is also based on their Granada Espada/ Wolfknights engine. Depending on how much they’ve been building off the existing framework, doing a multi-thread system to handle tasks could potentially be very expensive and buggy if they went that route now (as a game programmer, I’ve had nightmares with these scenarios). Although, most recent games either use an engine with such systems like Unity or Unreal, or do in-house engines with support for this. I doubt it was overlooked since multi-core is important nowadays, but it’s something to keep in mind.
Another thing that people should know is when framerates drop in highly populated areas, this is a typical CPU bottleneck, as it’s trying to execute lots of code and scripts for all those people. Usually a CPU with higher clock rate (GHz value) will help with this, along with multi-threaded optimization, but one should always do benchmarks to determine if the problem is really the CPU or GPU.
For DX12, there’s still a considerable amount of work involved in using the new APIs and making sure everything looks the same if you’re adding the feature to use it, so I’m thinking that won’t come in till much later, or at all if they can achieve decent framerates with what they’re doing now. Hopefully everything will go well.

.