> the template string expression itself (resulting in a Template object) will take a long while to evaluate

Are you saying that calling:

     template = t”{fib_slow()}”

Yes. Just like with f-strings, the call to `fib_slow()` is extracted at compile-time, and occurs eagerly at run-time.

What t-strings offer over f-strings is the ability to control how the final string is put together from the calculated results. And the t-string doesn't have `__str__` - you have to explicitly pass it to a named formatting function to get a string. (There is `__repr__, but that's for debugging, of course.) So you can potentially reuse the same Template instance (created from the t-string) multiple times in different contexts to format the same information different ways.

Yes, fib_slow() will be evaluated immediately, just like it would be if you were using an f-string.