That chip is actually reasonably power efficient, not very far away from leading edge x86 assuming 180W TDP. At 120W, the RISC-V server is on par or even exceeds the power efficiency of the x86 system.

https://www.sifive.com/blog/sifive-empowering-a-new-era-of-d...

According to that graph, the P870 has 80% of the performance of Ice Lake.