there is a post here about this problem but that's 10 months old and has got no comments so here i go.
I've got a problem with swiftkey where i can write in english without any problem, but when i switch to korean it gets laggy all of a suddena nd it's quite unbearable to type.
I'm currently on SwiftKey 184.108.40.2064 on Huawei P10 Android 7.0 on latest firmware that Huawei provide.
Currently i'm suing GBoard with Korean enabled and that one doesn't get laggy when typing inkorean so it's most likely it's Korean layout on Swiftkey that's the problem
I've already uninstalled and reinstalled Swiftkey but since the keyboard coems as standard in Huawei phones it just uninstall the updates and reinstall the updates again.
I've also tried cleaning the cache.