What do you mean "per-app in safari"? I'd like to turn it on globally, with a single exception: I want to be able to continue using shared photos albums with my two best friends.

I don't care enough about JS performance or, more generally, the mobile web, to want to disable it on safari, or even parts of it.

You can disable lockdown mode in web views for specific apps. You do it in settings because those apps don’t have the usual Safari UI for configuring that.