Would you like to get acquainted with a Common Lisp implementation? Do you have free time and would like to start by some small tasks?
ECL is currently offering a simple way for you to do so. Thanks to Anton Vodonosov's cl-test-grid and Eric Marsden's benchmark collection, which are offered here http://ecls.sourceforge.net/reports-generated/ecl/index.html and here http://ecls.sourceforge.net/pictures/index_js.html , you may now see in real time what problems are left to be solved in terms both of compatibility and performance.
Usually, the things to be corrected are small details, like a core function that does not have an optimised inline expansion (SEARCH, FIND, etc), or some tiny mismatch between the expectations of library developers and what I, as an implementor of ECL, understood as standard. Many of those bugs or performance regressions are also easy to debug (*).
If you wish to contribute, please do so through the appropriate channels, which include the ECL mailing list or, most notably, the bug/patch/feature/support reporting database in Sourceforge (https://sourceforge.net/p/ecls/news/new)
(*) If you just want to optimise the compilation of certain functions, performance of CLOS, etc, etc, this can be done knowing only Common Lisp, for the Lisp->C compiler is based entirely on Common Lisp functions and relies on compiler macros for optimisations. For other tasks, some C skills and acquaintance with a C debugger is needed.