SBCL has gc hooks you might also use too, though I don't think it'll give you the duration of the just-finished gc.
Variable: *after-gc-hooks* [sb-ext]¶
You might be able to just increment a counter, and call (room) each time, and at least log the data (assocaited with the counter) for subsequent analysis.
Lispers know their time is too valuable not to use lisp.
That's the closest approximation (so far) of what I was looking for, though unfortunately I'm a slime user and haven't yet tried sly.
I'm actually surprised there isn't speedbar for CL definitions already in slime, somehow I figured it was an obvious thing I was missing, since you see that sort of thing all the time in IDE's.
Nice tips for future use though not quite what I'm looking for (though the prefix tip is one I'll have to remember).
Are the sly presentations in a speedbar or special buffer of some kind, or just listed in the REPL?
You might read docs about some CL based games, like Khandria, where they talk about dealing with gc. (TL;DR, not a major problem). Sorry, I don't have a link for you.
Note that you can also work to writing less consing-intensive code too (more preallocated data structures, efficient use of fixnums, the stack, and lisp-specific extensions that allow you to do more tricks with the stack or
malloc()'dmemory.