ECL is obviously not the fastest Lisp (compared to SBCL), but compiled it's faster than I would have expected.
What I really like about ECL is that you can run e.g. EQL5 (ECL + Qt5) through a debugger (gdb in my case), so on crashes it's quite easy to find the offending code.
Even if there is some seg.fault in your Lisp/C integration, most of the time you can just :c (continue) from the break in the ECL debugger: it's not bringing down your whole application!
Since the EQL5 ECL/Qt5 integration is relatively low-level, it's quite fast (CommonQt with ECL is much slower; but CommonQt with SBCL is fast of course).
Additionally, EQL5 offers a QtWebkit/Lisp bridge, and better yet, a QML/Lisp bridge (which means, not only connecting QML and Qt signals and slots, which is possible in CommonQt, too), but convenient, direct(!) Lisp function calls from your QML code.
But that's just my small island where I live on (I mean both ECL and EQL5). Of course most people don't want to lock themselves to a single CL implementation...