> Which risc-v implementation is considered fast?

SpacemiT K3 is 2010 Macbook performance single-core, 2019 Macbook Air multi-core, and better than M4 Apple Silicon for AI.

So I guess it depends on what you are going to do with it.

Nothing shipping today is really competitive with modern ARM or x86. The SiFive P870 and Tenstorrent Ascalon (Jim Keller's team) are the most anticipated high-performance designs, but neither is widely available. What you can actually buy today tops out around Cortex-A76 class single-thread performance at best, which is roughly where ARM was five or six years ago.

DC-ROMA 2 is on the Rasperry 4 level of performance last I heard