just built this kernel 6.9.4-rt5, it is rt patched but not a fully preemptible realtime kernel, it is the next option down, tweakable automatic preemption. Overlay and aufs, non-usrmerge. Built with SUKK, but userspace aufs utils added manually post compile. I noticed in during make menuconfig a lot more ChromeOS hardware options, and I am on an Acer 714 (not spin), touchscreen (raydium), intel sst kabylake chipset, chromebook. And the difference is positively sporty. It seems to be 30% faster! I will have to do some benchmarks to make sure I am not exaggerating, against the previous 5.2.21-rt15 kernel I have been running for years. There are two new problems though: 1) the intel wifi card is being blocked by some module. 2) the SST soundcard is also being blocked by some other module. This was also the case with the previous kernel but I knew which modules to blacklist. So I will use my new usbc headset with latest 384khz conexant cx31993HiFi codec and amp chip on board, which sounds great. Also I will just use usbc tethering with the phone until I discover which modules to blacklist.
EDIT: wifi problem was simply missing firmware.