In my opinion it is not the performance of JavaScript or WebAssembly that is the real problem, it has not been for the past few years, it is the performance of the entire web stack (e.g. electron) that is the problem.
Combining all of the technologies in the web stack has performance hit far higher than 20%.
One of things I find myself having to teach people over and over again about performance:
It's like personal finance. At the end of the day you are not going to 'balance the checkbook' by identifying the 2 places where you are misbehaving the worst. You have to get every aspect of your spending below a % of your overall budget or you will always be in debt. Starting with the big things can be very motivational but it's not a strategy that works (or rather, it works by accident).
The relevant part for this discussion is that if you fix the slowest thing, (interpreting code) then the effects of all of the other sources of slowness are magnified. Everything else gets 'slower' by comparison (if you fix the thing that takes 50% of the time, then all of the things that used to take single digit percentages jump to double digits). You'll eventually get near your goal that way but the entire process is a game of whack-a-mole, and nobody will ever support you going back for the last 2% in some section of the code, leading to death by a thousand cuts (and honestly, I've watched a lot of people interpret performance reports and most of us can't even spot problems that small.)
Combining all of the technologies in the web stack has performance hit far higher than 20%.