In the words of the engineers who built the TurboFan optimizing compiler,
The problem that the engineers and developers are aware of is the fact that some platforms like the iOS, smart TVs, and game consoles don’t allow write access to memory for third party developers, so JIT becomes a moot feature. In that case, having an option to switch JIT off can allow for better accessibility when developing for platforms that don’t support JIT.
“WebAssembly is currently unsupported, but interpretation is also in the realm of possibility. V8’s builtins are still compiled to native code, but are no longer part of the managed JS heap, thanks to our recent efforts to embed them into the V8 binary.”
After testing JIT-less mode versus JIT mode on Youtube TV, the engineers found that JIT-less mode showed a 6% performance hit as compared to the standard configuration. The V8 team acknowledged that the performance regression was small relative to the 40% performance hit on Speedometer 2.0, leading them to conclude that testing environments don’t always translate to real world results.
image source: V8
Though one can also argue that Youtube TV is just one example and that further testing may reveal larger performance hits depending on the structure of the app. In any case, V8 has provided an alternative for those willing to work on restricted platforms. The performance hit might be negligible enough to try out JIT-less V8.