> And the expected performance for optimal `f'X is {x}'` code would 
> be *faster* than `"'X is %s' % (x,)"` which still needs to 
> interpret the string at runtime, and build a proper tuple object 
> on stack.

That's not necessarily true. The f-string version still needs to invoke the .format() method on the object, instead of only working for a handful of hard-coded types.

I'm not saying there aren't optimization opportunities, but it may be that %-formatting is always faster.