Looks like they are working on it, which is good news ->

From https://twitter.com/steipete/status/1013035117076467713 :

Shoutout to the Safari team! They were able to track down the slowness in the current WebAssembly implementation and ALREADY FIXED IT. Patch is here: https://trac.webkit.org/changeset/233378/webkit

(trace points caused a significant (4x!) slowdown as our benchmark performs many calls into the VM)