At the moment, even if you set up SSD for foreground operations and HDD only for background, more often then not, your write latencies are those of the HDD. We can do better.
See https://paquari.com/posts/bcachefs-ssd-hdd-progress/ for a progress report. (Disclosure: the write-up is an AI summary of my notes, so don't expect any brilliant prose.)
I'm also working on resurrecting stabilizer (https://github.com/matthiasgoergens/stabilizer). Stabilizer is a way to make program performance less dependent on the linker and layout lottery. But it has sadly fallen victim to bitrot.
I'm doing some minor contributions to other parts of Linux like ZFS and ext4, mostly as a byproduct of experiments I'm running for my bcachefs work. Another somewhat ambitions project is to make swapfiles on ZFS and bcachefs work (or work better), at the moment the problem is that writing swap on them might need to allocate memory exactly when you are out of memory.