I have the same issue here. Turned on chrome for it. The uploading isnt the only problem. Webflow designer is also consuming my memory. I have 20 gig in my imac and its getting the zero in several minutes. then it stops working before the cache cleared by an automated app. very annoying.
I’m also on Safari 11 for the last month. It was good in the earlier public betas of macOS HS. But now its up to date. but still same problem.