Poking through Wikipedia's entry on Colossus led me to
http://www.bletchleypark.org.uk/news/docview.rhtm/487682 , which has this quote: "My laptop digested ciphertext at a speed of 1.2 million characters per second – 240 times faster than Colossus. If you scale the CPU frequency by that factor, you get an equivalent clock of 5.8 MHz for Colossus. That is a remarkable speed for a computer built in 1944."
It should be pointed out that a linear scaling is inappropriate, though, because Colossus was not Turing complete, which makes direct comparisons very tricky. The very oldest things we today call "computers" tended to do a lot of stuff in very, very specialized hardware, and converting that into a modern-day, very general-purpose "clock cycle" is not trivial. It might be better to understand this as a glorified GPU-like processor. Only even less powerful, as it was even more special-purpose than a modern GPU.
(Or at least it will be better, if you understand the intrinsic limitations of GPU-like processing. If you're a Cell fanboy who bought the Sony's PS3 gibberish about performance hook line and sinker and still haven't worked out why Cell-based computers haven't destroyed Intel without invoking some sort of paranoid theory, that might not be as helpful a way of understanding things as I'd like.)