Can't say for sure but the way I read them most complaints about Javascript performance are about the results of the abuse of Javascript applications everywhere.

I guess Javascript just happens to be the common thing between a number of them, and people would be just as annoyed if the multi-megabyte, cpu-hogging, data-stealing monstrosities where hand-crafted in assembly ;-)

And: If someone complained about Javascript being slow for scientific calculations I'd probably call them out on it ;-)