Literally any Javascript benchmark in Lockdown vs not Lockdown will tell you. Their performance is horrid without JIT, because they’re nothing but Javascript compute in hot loops - the sort of thing JIT is ideal for.
At least on iOS, this is how Lockdown mode works. It disables all the stuff, but you can go to the settings for a specific site (domain) and disable Lockdown - you get everything back, not just JIT, so webfonts, WebGL, etc. But it’s a per-domain toggle.