I'd say right in the early 2000s and then Django starts becoming popular, and the rest is history. But the writing is really on the wall in the late 1990s due to the intuitiveness of the Python language and beginner friendly design compared to the other popular scripting languages at the time including Perl, TCL, PHP, Ruby, etc. Speed, however, is never Python's advantage and for scripting languages it's not really a factor.

I think based on Python background it's safe to say that in order to become a hugely popular programming language, any language need to be beginner friendly and/or need a killer application as a motivation for programmers to adopt it. Speed and performance are not important factors unless you are writing an OS and we have already C for that.