Making multi-threaded applications is extremely difficult, even more so with things like replaying a blockchain since every block relies on the previous one to be correct.
Check this post for more info: https://www.quora.com/Why-is-multi-threading-so-damn-hard?share=1
I hope that the developers can improve this in some way. It's possible many of the issues are already solved in EOS, but may never make it to light in Steem.