Hi Crinity,
Unfortunately, there isn’t much I can report at this juncture; there are a couple of Metal related issues that I know are being investigated, however neither are slated to for fixing prior to the 4.9 release, and even that is not at all a certainty. The only workaround we can see right now is that the perf drop does not seem to occur if Metal is disabled in the editor.
I will definitely update this thread when I have more information.