Intrusive vs. not-intrusive isn't nearly sufficient to make that comparison. For example, in phkmalloc the switch to a separate index generally improved performance relative to the allocator being replaced: http://phk.freebsd.dk/pubs/malloc.pdf However, from that paper it seems that the previous allocator made some particularly bad choices that resulted in unnecessarily poor cache locality.

OpenBSD's allocator uses a separate index, as well as many more mitigations (many more than Slitter), and unfortunately OpenBSD's allocator ends up being substantially, even notoriously slower than most other allocators. But, again, this has little to do with using a separate index, except that using a separate index opens the door to many other potential mitigations. And where ever there's a choice between performance or mitigation, OpenBSD usually takes the latter (though sometimes they walk that back as usage and efficacy become more clear).