My feeling is it shouldn’t block the rollout. Not only is JVM Scala much bigger, I would also argue:
The use cases are also a lot more performance sensitive. Frontend code tends to be more limited by complexity than perf
And a lot of browser work that is performance sensitive is not bound on Scala.js execution speed: script download speed, script parsing speed, JS library speed (e.g. React), network round trips (front code tends to be ajaxy), DOM perf, CSS rendering perf (my personal most recent Scala.js perf battle)
Browser perf is a mercurial thing. While the JVM behaves the same yesterday as today, browser optimizations change rapidly and perf for the same code can swing around massively over time, nevermind different browsers. IME the standard perf advice is to try to optimize code “semantically”, but not to micro-optimize with the browser optimizer in mind as such wins or losses tend to be fleeting.