Afaik the StarFive SOCs used in SBCs are a lot slower than current ARM offerings. Part of that might be because software support is worse, so maybe compilers and related tooling aren't yet optimized for them?
Hopefully development on these continues to improve though. The biggest nail in the coffin for Pi alternatives has been software support.
It's also a super new architecture and completely open source. Development will simply be slower. There's enough "mainstream" Pi competitors with risc-v offerings though that I'm confident that it will happen.
STAR64 is an ARM board. Pine64 is definitely eyeing RISC-V heavily but last I checked the only product that actually shipped with a RISC-V CPU is the pinecil featuring a SiFive E24, a beefy microcontroller. Ridiculously beefy for a soldering iron, but a microcontroller nonetheless.
Killing off ARM in that segment is still some ways off but RISC-V already has a firm foothold in the microcontroller space. Also Arduinos are absolutely overpriced which doesn't help them a bit.
The Quartz64 Model A is a single-board computer featuring the Rockchip RK3566 SoC. This SoC combines a quad-core, ARM Cortex-A55 CPU with a Mali-G52-2EE GPU.
...as said, I haven't kept up to date. Don't shoot the messenger shoot Pine64's webdev.
The Pinecil was their first Risc-V product and the first commercial product with Risc-V made by anyone.
As a dedicated and user-programmable device, probably yes but e.g. Seagate has been shipping HDDs with RISC-V controllers for ages. Apparently they want/need custom silicon and paying ARM for a CortexM0 license was just something they decided wasn't necessary, and actually annoying as you can't easily extend the ARM to include custom instructions, ARM is very protective of compatibility and everything.
There are a lot of cheap Arduino UNO and NANO knockoffs and the tech is so stupidly simple (and ancient) it doesn't take much to design your own (you can put together the most of the hw on a breadboard as long as you get an extrenal USB-Serial adaptor module, and those are also cheap as chips)
For the more modern and powerful stuff at that end of the computing power (i.e. the low, low, oh-so-low end) the simplest option is an ESP8266 or ESP32, which you can get as a module for about $3 - $5 or as a board for about 2x to 3x the price.
That said, this stuff are microcontrollers, not microprocessors, and they're nowhere in the same range as SBCs like the Pi, so I wouldn't advise using one of those if what you want is something that can run Linux and use displays with more resolution than 128x80.