From benchmarks I've seen elsewhere, native code is generally roughly 2x faster than WASM, so I doubt that's the case. I'm sure it depends on a lot of factors, though.

Single-threaded, non-SIMD x86-64 code being 1.2x to 2x faster than WASM is what I'm seeing as well. Interestingly I don't see such big differences between browsers as the PSPDFKit benchmark, my suspicion is that this mainly measures overhead when crossing the WASM-JS border.