you are lucky: @hayley worked on the SBCL GC, so she can answer.
My 2cents are that, apart the GC, CL gives you a lot of power for optimizing memory management because: with MOP you can customize the memory layout of CLOS objects; if performances are important, you can work on big-chunk of data, stored in arrays, and here CL can give you a lot of options for specifying their format. In both cases, you are reducing the work for the GC, because you have less objects, and you can control (at least partially) how they are stored.
It is a lot of power, that few PL offers.
@ramin_hal9001@emacs.ch @screwtape @awkravchuk