That paper does not report "WebAssembly is running at 67% of native speed in Firefox and 53% of native speed in Chrome".
That paper reports — "… applications compiled to WebAssembly run slower by an average of 50% (Firefox) to 89% (Chrome), with peak slowdowns of 2.6× (Firefox) and 3.14× Chrome)."
When you write "53% of native speed" that's really confusing!
That paper reports — "… applications compiled to WebAssembly run slower by an average of 50% (Firefox) to 89% (Chrome), with peak slowdowns of 2.6× (Firefox) and 3.14× Chrome)."
When you write "53% of native speed" that's really confusing!