Because pg_jitter uses AsmJit's Compiler, which also allocates registers. That's much more work than using hardcoded physical registers in SLJIT case. There is always a cost of such comfort.
I think AsmJit's strength is completeness of its backends as you can emit nice SIMD code with it (like AVX-512). But the performance could be better of course, and that's possible - making it 2x faster would be possible.
I think AsmJit's strength is completeness of its backends as you can emit nice SIMD code with it (like AVX-512). But the performance could be better of course, and that's possible - making it 2x faster would be possible.