Last spring, the Maxima challenge for ABCL was to get it to complete its test suite. We've mastered that bit of Maxima for some months now. However, as turned out soon after that, Maxima runs rather slow on ABCL. To some extent - being limited by the JVM - that's to be expected. The performance observed was way off base though: much too slow.
Through analysis, the cause was established to be the fact that Maxima declares lots of symbols to be special [and that ABCL doesn't offer a way to remove that specialness].
Last summer we found that allowing Maxima to undeclare specials increases ABCL performance immensely (roughly 35%). However, the final goal for Maxima is to make more sparingly use of specials and declaring unspecial or undeclaring special isn't defined in the spec. Because of the two reasons it felt not right to implement the solution at the time: it would have been a very Maxima specific one to a problem Maxima intends to fix in the long run.
Peter Graves noted that he converted the special bindings storage in XCL to use the same scheme as in SBCL/CCL, using an array with active bindings instead of a linked list of bindings. He observed a performance gain of 10% in his tests.
Last weekend, I implemented the same scheme in ABCL and although the general speed up doesn't show 10% in our tests [which may very well differ from Peter's], we observed roughly 40% performance gain on Maxima's test suite!