I am not sure as to what could be causing this issue.
Would it be possible to upload your file to a shared location and post the link here for investigation.
Usually slow playback is due to not enough memory. Clean out junk files on your computer, close background programs, just try to free up memory.
It really is over all files, so don't think it is specifically the one I'm using.
But it did get a bit better after the latest update of Animate CC 2018. Giving it some more time.
I have 128 GB of memory, so would think that is enough? Could be having After Effects open at the same time... Memory hog no. #1.