I personally think that might depend on what you mean by "slow". Tcl/Tk programs can be very light and memory-efficient -- you can pack a lot of features into a program that only takes 10 megabytes or less at runtime and that starts up and responds to input near-instantly. The event loop abstraction which is well-integrated in Tcl makes it easy to handle sockets, files, timers, and gui events all on one loop which done right can keep your program very responsive without needing to worry about multithreading.

In terms of pure runtime performance (CPU efficiency I guess), do you know of any reasonable benchmarks comparing TCL to say, Python and Ruby? I wonder if they might all be in the same ballpark?