Compared to similar dynamic scripting languages, Q is very vast. Compared to statically compiled languages, it can be surprisingly competitive, but is usually slower. The truly distinctive thing about Q is its efficiency as a user interface: at a REPL you can rattle off a short sequence of characters to transform and interrogate large datasets at interactive speeds and flexibly debug complex distributed systems live. In the right hands, it's a stunningly effective rapid-application-development tool (the above "quant desk scenario"); this was perhaps even more true in the k2 days when it was possible to build ugly but blisteringly fast and utilitarian data-bound GUIs for K programs in a few lines of code. There's certainly an abundance of romanticism and mythology surrounding it, but some of the claims are real and enduringly unmatched.
1. it was fast by a huge margin for its time
2. the reason for its speed is the language behind it
3. it uses an esoteric language and still attains success
4. the core engine is implemented using surprisingly few lines of code
5. the core has been written and maintained by one person
All of these are things I've heard so I can't claim it's 100% true but I'm sure it's a combination of some of these.
I feel like APL and all its relatives had long ago gained legendary status. So the legend lives on - maybe longer than it should.
Don't get me wrong. It's still amazing!