It's hard to tell at this point, but it should be close to or even overall better than Pi 4 / Pi 400 performance -- worse CPU (but better than Pi 3+), but better and more RAM and better I/O e.g. M.2 SSD. And whatever real GPU you want to put on it instead of the fairly low end one on the Pi 4 -- SiFive are demonstrating their board with the Radeon RX580, a 150W $300 GPU but lower end ones should work fine too.

All at a much higher price than a Pi, of course, but similar to x86 PCs.

Compared to x86 PCs the CPU performance is probably around 2001 or so Pentium III or PPC G4 but, again, with everything else (16 GB DDR4 RAM, M.2 SD, GPU of your choice) current generation.