Should doing arithmetic quickly in ones head be considered a virtue? I wish math classes would focus more on the abstraction aspect and less on calculation.

Not that I disagree about wanting more of the abstraction taught, but absolutely, 100% yes. Accurate, rapid mental math is a core life skill, and the lack of it causes people real problems.

There are many people who are bad/slow at calculation and amazing at math; many of them have math PhDs even. Ya, they might use a calculator in their daily life, but those are cheap anyways.

It would be nice to get an ALU implanted in my brain, it would be much more efficient for calculation than using a neural network.

Mental math skills are for prosaic daily-life problems, and have almost nothing to do with mathematics as a discipline. (Being good at spelling on its own does not make one a good writer.) Not needing a calculator at the grocery store to figure out which of some competing products are cheaper per unit mass, or figure out change + tip at a restaurant is useful knowledge.

Yes, I think it should be considered a virtue. It's a very useful skill that I used daily.

It shouldn't be beaten into kids ("learn your timestables!!" is not an effective teaching method), but if using an abacus or similar helps people calculate faster, it is likely a good thing.

I am not sure, but doesn't it make also sense financially for some parents to send their kids to soroban classes because that's actually cheaper and more frequent in a week than other alternatives when they cannot go anymore to the public after-school services (I speak about gakudo/学童 here)?