https://discord.com/invite/Ck7jw53nU2 - Zephyr just recently moved to Discord, and anyone can join AFAIK. It's not limited to the members. BTW Freescale merged into NXP, that's why it's not on any list any more :)

Added this to the article - it's 100 MHz in this design. For some more performance info on Vex in general, see https://github.com/SpinalHDL/VexRiscv

We've yet to make detailed analyses on the multicore version, but in general I'd say it's pretty decent.