Ah so it seems. Somehow I thought __format__ was slotted, but that is not the case and it needs to be looked up, and what is worse, of course a tuple needs to be built as well. 

Oh well, at least it should be quite minimal to make it be faster than `f(x)` there, which necessarily has one extra bound method call and interpretation of format string as the overhead, so there's minimally at least 30 % performance boost to achieve.