Last time I tried the full disk encyrption [sic] it was an awful setup compared to Linux. > https://why-openbsd.rocks/fact/meltdown-spectre/ Uh, yeah. They did that, just like Linux did before them. I especially like the reply to the announcement that was "uh… I hope you didn't spend these two months coming up with that solution. We already did that for Linux, so you could have just asked".
Just because the idea is known doesn't make it trivial to implement in a completely different kernel.
It appears that OpenBSD chose to spend months starting from scratch researching exactly what needed to be mapped in virtual memory, instead of just confirming the x86/x64 platform needs that were already researched and published by that time.